Add support for GFM footnotes

19 files changed, 3058 insertions, 148 deletions

diff --git a/readme.md b/readme.md
index 565d90a..b4788ae 100644
--- a/readme.md
+++ b/readme.md
@@ -141,7 +141,7 @@ They are not enabled by default but can be turned on with `options.constructs`.
 - [x] frontmatter
 - [ ] gfm
   - [x] autolink literal
-  - [ ] footnote
+  - [x] footnote
   - [x] strikethrough
   - [ ] table
   - [ ] tagfilter
diff --git a/src/compiler.rs b/src/compiler.rs
index f1003fd..6ad34b2 100644
--- a/src/compiler.rs
+++ b/src/compiler.rs
@@ -18,7 +18,8 @@ use alloc::{
 };
 use core::str;
 
-/// Link or image, resource or reference.
+/// Link, image, or footnote call.
+/// Resource or reference.
 /// Reused for temporary definitions as well, in the first pass.
 #[derive(Debug)]
 struct Media {
@@ -76,6 +77,8 @@ struct CompileContext<'a> {
     pub events: &'a [Event],
     /// List of bytes.
     pub bytes: &'a [u8],
+    /// Configuration.
+    pub options: &'a Options,
     // Fields used by handlers to track the things they need to track to
     // compile markdown.
     /// Rank of heading (atx).
@@ -100,6 +103,10 @@ struct CompileContext<'a> {
     pub tight_stack: Vec<bool>,
     /// List of definitions.
     pub definitions: Vec<Definition>,
+    /// List of definitions.
+    pub gfm_footnote_definitions: Vec<(String, String)>,
+    pub gfm_footnote_definition_calls: Vec<(String, usize)>,
+    pub gfm_footnote_definition_stack: Vec<(usize, usize)>,
     // Fields used to influance the current compilation.
     /// Ignore the next line ending.
     pub slurp_one_line_ending: bool,
@@ -128,7 +135,7 @@ impl<'a> CompileContext<'a> {
     pub fn new(
         events: &'a [Event],
         bytes: &'a [u8],
-        options: &Options,
+        options: &'a Options,
         line_ending: LineEnding,
     ) -> CompileContext<'a> {
         CompileContext {
@@ -143,6 +150,9 @@ impl<'a> CompileContext<'a> {
             list_expect_first_marker: None,
             media_stack: vec![],
             definitions: vec![],
+            gfm_footnote_definitions: vec![],
+            gfm_footnote_definition_calls: vec![],
+            gfm_footnote_definition_stack: vec![],
             tight_stack: vec![],
             slurp_one_line_ending: false,
             image_alt_inside: false,
@@ -161,6 +171,7 @@ impl<'a> CompileContext<'a> {
             allow_dangerous_html: options.allow_dangerous_html,
             buffers: vec![String::new()],
             index: 0,
+            options,
         }
     }
 
@@ -243,6 +254,11 @@ pub fn compile(events: &[Event], bytes: &[u8], options: &Options) -> String {
     //
     // To speed things up, we collect the places we can jump over for the
     // second pass.
+    //
+    // We don’t need to handle GFM footnote definitions like this, because
+    // unlike normal definitions, what they produce is not used in calls.
+    // It would also get very complex, because footnote definitions can be
+    // nested.
     while index < events.len() {
         let event = &events[index];
 
@@ -250,15 +266,15 @@ pub fn compile(events: &[Event], bytes: &[u8], options: &Options) -> String {
             handle(&mut context, index);
         }
 
-        if event.name == Name::Definition {
-            if event.kind == Kind::Enter {
+        if event.kind == Kind::Enter {
+            if event.name == Name::Definition {
                 handle(&mut context, index); // Also handle start.
                 definition_inside = true;
                 definition_indices.push((index, index));
-            } else {
-                definition_inside = false;
-                definition_indices.last_mut().unwrap().1 = index;
             }
+        } else if event.name == Name::Definition {
+            definition_inside = false;
+            definition_indices.last_mut().unwrap().1 = index;
         }
 
         index += 1;
@@ -278,14 +294,17 @@ pub fn compile(events: &[Event], bytes: &[u8], options: &Options) -> String {
             jump = definition_indices
                 .get(definition_index)
                 .unwrap_or(&jump_default);
-            // Ignore line endings after definitions.
-            context.slurp_one_line_ending = true;
         } else {
             handle(&mut context, index);
             index += 1;
         }
     }
 
+    // No section to generate.
+    if !context.gfm_footnote_definition_calls.is_empty() {
+        generate_footnote_section(&mut context);
+    }
+
     assert_eq!(context.buffers.len(), 1, "expected 1 final buffer");
     context
         .buffers
@@ -312,6 +331,7 @@ fn enter(context: &mut CompileContext) {
         | Name::CodeFencedFenceMeta
         | Name::DefinitionLabelString
         | Name::DefinitionTitleString
+        | Name::GfmFootnoteDefinitionPrefix
         | Name::HeadingAtxText
         | Name::HeadingSetextText
         | Name::Label
@@ -326,6 +346,8 @@ fn enter(context: &mut CompileContext) {
         Name::DefinitionDestinationString => on_enter_definition_destination_string(context),
         Name::Emphasis => on_enter_emphasis(context),
         Name::Frontmatter => on_enter_frontmatter(context),
+        Name::GfmFootnoteDefinition => on_enter_gfm_footnote_definition(context),
+        Name::GfmFootnoteCall => on_enter_gfm_footnote_call(context),
         Name::GfmStrikethrough => on_enter_gfm_strikethrough(context),
         Name::GfmTaskListItemCheck => on_enter_gfm_task_list_item_check(context),
         Name::HtmlFlow => on_enter_html_flow(context),
@@ -374,6 +396,12 @@ fn exit(context: &mut CompileContext) {
         Name::GfmAutolinkLiteralProtocol => on_exit_gfm_autolink_literal_protocol(context),
         Name::GfmAutolinkLiteralWww => on_exit_gfm_autolink_literal_www(context),
         Name::GfmAutolinkLiteralEmail => on_exit_gfm_autolink_literal_email(context),
+        Name::GfmFootnoteCall => on_exit_gfm_footnote_call(context),
+        Name::GfmFootnoteDefinitionLabelString => {
+            on_exit_gfm_footnote_definition_label_string(context);
+        }
+        Name::GfmFootnoteDefinitionPrefix => on_exit_gfm_footnote_definition_prefix(context),
+        Name::GfmFootnoteDefinition => on_exit_gfm_footnote_definition(context),
         Name::GfmStrikethrough => on_exit_gfm_strikethrough(context),
         Name::GfmTaskListItemCheck => on_exit_gfm_task_list_item_check(context),
         Name::GfmTaskListItemValueChecked => on_exit_gfm_task_list_item_value_checked(context),
@@ -472,6 +500,23 @@ fn on_enter_frontmatter(context: &mut CompileContext) {
     context.buffer();
 }
 
+/// Handle [`Enter`][Kind::Enter]:[`GfmFootnoteDefinition`][Name::GfmFootnoteDefinition].
+fn on_enter_gfm_footnote_definition(context: &mut CompileContext) {
+    context.tight_stack.push(false);
+}
+
+/// Handle [`Enter`][Kind::Enter]:[`GfmFootnoteCall`][Name::GfmFootnoteCall].
+fn on_enter_gfm_footnote_call(context: &mut CompileContext) {
+    context.media_stack.push(Media {
+        image: false,
+        label_id: None,
+        label: None,
+        reference_id: None,
+        destination: None,
+        title: None,
+    });
+}
+
 /// Handle [`Enter`][Kind::Enter]:[`GfmStrikethrough`][Name::GfmStrikethrough].
 fn on_enter_gfm_strikethrough(context: &mut CompileContext) {
     if !context.image_alt_inside {
@@ -961,6 +1006,92 @@ fn on_exit_gfm_autolink_literal_email(context: &mut CompileContext) {
     on_exit_autolink_email(context);
 }
 
+/// Handle [`Exit`][Kind::Exit]:[`GfmFootnoteCall`][Name::GfmFootnoteCall].
+fn on_exit_gfm_footnote_call(context: &mut CompileContext) {
+    let indices = context.media_stack.pop().unwrap().label_id.unwrap();
+    let id =
+        normalize_identifier(Slice::from_indices(context.bytes, indices.0, indices.1).as_str());
+    let safe_id = sanitize_uri(&id.to_lowercase(), &None);
+    let mut call_index = 0;
+
+    // See if this has been called before.
+    while call_index < context.gfm_footnote_definition_calls.len() {
+        if context.gfm_footnote_definition_calls[call_index].0 == id {
+            break;
+        }
+        call_index += 1;
+    }
+
+    // New.
+    if call_index == context.gfm_footnote_definition_calls.len() {
+        context.gfm_footnote_definition_calls.push((id, 0));
+    }
+
+    // Increment.
+    context.gfm_footnote_definition_calls[call_index].1 += 1;
+
+    // No call is output in an image alt, though the definition and
+    // backreferences are generated as if it was the case.
+    if context.image_alt_inside {
+        return;
+    }
+
+    context.push("<sup><a href=\"#");
+    if let Some(ref value) = context.options.gfm_footnote_clobber_prefix {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("user-content-");
+    }
+    context.push("fn-");
+    context.push(&safe_id);
+    context.push("\" id=\"");
+    if let Some(ref value) = context.options.gfm_footnote_clobber_prefix {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("user-content-");
+    }
+    context.push("fnref-");
+    context.push(&safe_id);
+    if context.gfm_footnote_definition_calls[call_index].1 > 1 {
+        context.push("-");
+        context.push(
+            &context.gfm_footnote_definition_calls[call_index]
+                .1
+                .to_string(),
+        );
+    }
+    context.push("\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">");
+
+    context.push(&(call_index + 1).to_string());
+    context.push("</a></sup>");
+}
+
+/// Handle [`Exit`][Kind::Exit]:[`GfmFootnoteDefinitionLabelString`][Name::GfmFootnoteDefinitionLabelString].
+fn on_exit_gfm_footnote_definition_label_string(context: &mut CompileContext) {
+    context
+        .gfm_footnote_definition_stack
+        .push(Position::from_exit_event(context.events, context.index).to_indices());
+}
+
+/// Handle [`Exit`][Kind::Exit]:[`GfmFootnoteDefinitionPrefix`][Name::GfmFootnoteDefinitionPrefix].
+fn on_exit_gfm_footnote_definition_prefix(context: &mut CompileContext) {
+    // Drop the prefix.
+    context.resume();
+    // Capture everything until end of definition.
+    context.buffer();
+}
+
+/// Handle [`Exit`][Kind::Exit]:[`GfmFootnoteDefinition`][Name::GfmFootnoteDefinition].
+fn on_exit_gfm_footnote_definition(context: &mut CompileContext) {
+    let value = context.resume();
+    let indices = context.gfm_footnote_definition_stack.pop().unwrap();
+    context.tight_stack.pop();
+    context.gfm_footnote_definitions.push((
+        normalize_identifier(Slice::from_indices(context.bytes, indices.0, indices.1).as_str()),
+        value,
+    ));
+}
+
 /// Handle [`Exit`][Kind::Exit]:[`GfmStrikethrough`][Name::GfmStrikethrough].
 fn on_exit_gfm_strikethrough(context: &mut CompileContext) {
     if !context.image_alt_inside {
@@ -1080,7 +1211,12 @@ fn on_exit_label_text(context: &mut CompileContext) {
 fn on_exit_line_ending(context: &mut CompileContext) {
     if context.code_text_inside {
         context.push(" ");
-    } else if context.slurp_one_line_ending {
+    } else if context.slurp_one_line_ending
+        // Ignore line endings after definitions.
+        || (context.index > 1
+            && (context.events[context.index - 2].name == Name::Definition
+                || context.events[context.index - 2].name == Name::GfmFootnoteDefinition))
+    {
         context.slurp_one_line_ending = false;
     } else {
         context.push(&encode(
@@ -1113,9 +1249,12 @@ fn on_exit_list_item(context: &mut CompileContext) {
         context.index - 1,
         &[
             Name::BlankLineEnding,
+            Name::BlockQuotePrefix,
             Name::LineEnding,
             Name::SpaceOrTab,
-            Name::BlockQuotePrefix,
+            // Also ignore things that don’t contribute to the document.
+            Name::Definition,
+            Name::GfmFootnoteDefinition,
         ],
     );
     let previous = &context.events[before_item];
@@ -1167,7 +1306,6 @@ fn on_exit_media(context: &mut CompileContext) {
 
     let media = context.media_stack.pop().unwrap();
     let label = media.label.unwrap();
-    let image_alt_inside = context.image_alt_inside;
     let id = media.reference_id.or(media.label_id).map(|indices| {
         normalize_identifier(Slice::from_indices(context.bytes, indices.0, indices.1).as_str())
     });
@@ -1190,7 +1328,7 @@ fn on_exit_media(context: &mut CompileContext) {
         None
     };
 
-    if !image_alt_inside {
+    if !is_in_image {
         if media.image {
             context.push("<img src=\"");
         } else {
@@ -1223,7 +1361,7 @@ fn on_exit_media(context: &mut CompileContext) {
         context.push(&label);
     }
 
-    if !image_alt_inside {
+    if !is_in_image {
         context.push("\"");
 
         let title = if let Some(index) = definition_index {
@@ -1248,7 +1386,7 @@ fn on_exit_media(context: &mut CompileContext) {
     if !media.image {
         context.push(&label);
 
-        if !image_alt_inside {
+        if !is_in_image {
             context.push("</a>");
         }
     }
@@ -1300,6 +1438,154 @@ fn on_exit_thematic_break(context: &mut CompileContext) {
     context.push("<hr />");
 }
 
+/// Generate a footnote section.
+fn generate_footnote_section(context: &mut CompileContext) {
+    context.line_ending_if_needed();
+    context.push("<section data-footnotes=\"\" class=\"footnotes\"><");
+    if let Some(ref value) = context.options.gfm_footnote_label_tag_name {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("h2");
+    }
+    context.push(" id=\"footnote-label\" ");
+    if let Some(ref value) = context.options.gfm_footnote_label_attributes {
+        context.push(value);
+    } else {
+        context.push("class=\"sr-only\"");
+    }
+    context.push(">");
+    if let Some(ref value) = context.options.gfm_footnote_label {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("Footnotes");
+    }
+    context.push("</");
+    if let Some(ref value) = context.options.gfm_footnote_label_tag_name {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("h2");
+    }
+    context.push(">");
+    context.line_ending();
+    context.push("<ol>");
+
+    let mut index = 0;
+    while index < context.gfm_footnote_definition_calls.len() {
+        generate_footnote_item(context, index);
+        index += 1;
+    }
+
+    context.line_ending();
+    context.push("</ol>");
+    context.line_ending();
+    context.push("</section>");
+    context.line_ending();
+}
+
+/// Generate a footnote item from a call.
+fn generate_footnote_item(context: &mut CompileContext, index: usize) {
+    let id = &context.gfm_footnote_definition_calls[index].0;
+    let safe_id = sanitize_uri(&id.to_lowercase(), &None);
+
+    // Find definition: we’ll always find it.
+    let mut definition_index = 0;
+    while definition_index < context.gfm_footnote_definitions.len() {
+        if &context.gfm_footnote_definitions[definition_index].0 == id {
+            break;
+        }
+        definition_index += 1;
+    }
+
+    debug_assert_ne!(
+        definition_index,
+        context.gfm_footnote_definitions.len(),
+        "expected definition"
+    );
+
+    context.line_ending();
+    context.push("<li id=\"");
+    if let Some(ref value) = context.options.gfm_footnote_clobber_prefix {
+        context.push(&encode(value, context.encode_html));
+    } else {
+        context.push("user-content-");
+    }
+    context.push("fn-");
+    context.push(&safe_id);
+    context.push("\">");
+    context.line_ending();
+
+    // Create one or more backreferences.
+    let mut reference_index = 0;
+    let mut backreferences = String::new();
+    while reference_index < context.gfm_footnote_definition_calls[index].1 {
+        if reference_index != 0 {
+            backreferences.push(' ');
+        }
+        backreferences.push_str("<a href=\"#");
+        if let Some(ref value) = context.options.gfm_footnote_clobber_prefix {
+            backreferences.push_str(&encode(value, context.encode_html));
+        } else {
+            backreferences.push_str("user-content-");
+        }
+        backreferences.push_str("fnref-");
+        backreferences.push_str(&safe_id);
+        if reference_index != 0 {
+            backreferences.push('-');
+            backreferences.push_str(&(reference_index + 1).to_string());
+        }
+        backreferences.push_str(
+            "\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"",
+        );
+        if let Some(ref value) = context.options.gfm_footnote_back_label {
+            backreferences.push_str(&encode(value, context.encode_html));
+        } else {
+            backreferences.push_str("Back to content");
+        }
+        backreferences.push_str("\">↩");
+        if reference_index != 0 {
+            backreferences.push_str("<sup>");
+            backreferences.push_str(&(reference_index + 1).to_string());
+            backreferences.push_str("</sup>");
+        }
+        backreferences.push_str("</a>");
+
+        reference_index += 1;
+    }
+
+    let value = context.gfm_footnote_definitions[definition_index].1.clone();
+    let bytes = value.as_bytes();
+    let mut byte_index = bytes.len();
+    // Move back past EOL.
+    while byte_index > 0 && matches!(bytes[byte_index - 1], b'\n' | b'\r') {
+        byte_index -= 1;
+    }
+    // Check if it ends in `</p>`.
+    // This is a bit funky if someone wrote a safe paragraph by hand in
+    // there.
+    // But in all other cases, `<` and `>` would be encoded, so we can be
+    // sure that this is generated by our compiler.
+    if byte_index > 3
+        && bytes[byte_index - 4] == b'<'
+        && bytes[byte_index - 3] == b'/'
+        && bytes[byte_index - 2] == b'p'
+        && bytes[byte_index - 1] == b'>'
+    {
+        let (before, after) = bytes.split_at(byte_index - 4);
+        let mut result = String::new();
+        result.push_str(str::from_utf8(before).unwrap());
+        result.push(' ');
+        result.push_str(&backreferences);
+        result.push_str(str::from_utf8(after).unwrap());
+        context.push(&result);
+    } else {
+        context.push(&value);
+        context.line_ending_if_needed();
+        context.push(&backreferences);
+    }
+    context.line_ending_if_needed();
+    context.push("</li>");
+}
+
 /// Generate an autolink (used by unicode autolinks and GFM autolink literals).
 fn generate_autolink(context: &mut CompileContext, protocol: Option<&str>, value: &str) {
     if !context.image_alt_inside {
diff --git a/src/construct/definition.rs b/src/construct/definition.rs
index e65d979..1d67635 100644
--- a/src/construct/definition.rs
+++ b/src/construct/definition.rs
@@ -175,14 +175,14 @@ pub fn label_after(tokenizer: &mut Tokenizer) -> State {
     tokenizer.tokenize_state.token_2 = Name::Data;
     tokenizer.tokenize_state.token_3 = Name::Data;
 
-    tokenizer.tokenize_state.end = skip::to_back(
-        &tokenizer.events,
-        tokenizer.events.len() - 1,
-        &[Name::DefinitionLabelString],
-    );
-
     match tokenizer.current {
         Some(b':') => {
+            tokenizer.tokenize_state.end = skip::to_back(
+                &tokenizer.events,
+                tokenizer.events.len() - 1,
+                &[Name::DefinitionLabelString],
+            );
+
             tokenizer.enter(Name::DefinitionMarker);
             tokenizer.consume();
             tokenizer.exit(Name::DefinitionMarker);
diff --git a/src/construct/document.rs b/src/construct/document.rs
index b438808..9c76e46 100644
--- a/src/construct/document.rs
+++ b/src/construct/document.rs
@@ -1,12 +1,13 @@
 //! The document content type.
 //!
-//! **Document** represents the containers, such as block quotes and lists,
-//! which structure the document and contain other sections.
+//! **Document** represents the containers, such as block quotes, list items,
+//! or GFM footnotes, which structure the document and contain other sections.
 //!
 //! The constructs found in flow are:
 //!
 //! *   [Block quote][crate::construct::block_quote]
 //! *   [List item][crate::construct::list_item]
+//! *   [GFM: Footnote definition][crate::construct::gfm_footnote_definition]
 
 use crate::event::{Content, Event, Kind, Link, Name};
 use crate::state::{Name as StateName, State};
@@ -99,6 +100,7 @@ pub fn container_existing_before(tokenizer: &mut Tokenizer) -> State {
 
         let name = match container.kind {
             Container::BlockQuote => StateName::BlockQuoteContStart,
+            Container::GfmFootnoteDefinition => StateName::GfmFootnoteDefinitionContStart,
             Container::ListItem => StateName::ListItemContStart,
         };
 
@@ -185,7 +187,7 @@ pub fn container_new_before(tokenizer: &mut Tokenizer) -> State {
 /// ```
 pub fn container_new_before_not_block_quote(tokenizer: &mut Tokenizer) -> State {
     // List item?
-    // We replace the empty block quote container for this new list one.
+    // We replace the empty block quote container for this new list item one.
     tokenizer.tokenize_state.document_container_stack
         [tokenizer.tokenize_state.document_continued] = ContainerState {
         kind: Container::ListItem,
@@ -200,14 +202,38 @@ pub fn container_new_before_not_block_quote(tokenizer: &mut Tokenizer) -> State
     State::Retry(StateName::ListItemStart)
 }
 
-/// At new container, but not a list (or block quote).
+/// At new container, but not a block quote or list item.
 //
 /// ```markdown
 /// > | a
 ///     ^
 /// ```
 pub fn container_new_before_not_list(tokenizer: &mut Tokenizer) -> State {
-    // It wasn’t a new block quote or a list.
+    // Footnote definition?
+    // We replace the empty list item container for this new footnote
+    // definition one.
+    tokenizer.tokenize_state.document_container_stack
+        [tokenizer.tokenize_state.document_continued] = ContainerState {
+        kind: Container::GfmFootnoteDefinition,
+        blank_initial: false,
+        size: 0,
+    };
+
+    tokenizer.attempt(
+        State::Next(StateName::DocumentContainerNewAfter),
+        State::Next(StateName::DocumentContainerNewBeforeNotGfmFootnoteDefinition),
+    );
+    State::Retry(StateName::GfmFootnoteDefinitionStart)
+}
+
+/// At new container, but not a block quote, list item, or footnote definition.
+//
+/// ```markdown
+/// > | a
+///     ^
+/// ```
+pub fn container_new_before_not_footnote_definition(tokenizer: &mut Tokenizer) -> State {
+    // It wasn’t a new block quote, list item, or footnote definition.
     // Swap the new container (in the middle) with the existing one (at the end).
     // Drop what was in the middle.
     tokenizer
@@ -227,7 +253,7 @@ pub fn container_new_before_not_list(tokenizer: &mut Tokenizer) -> State {
 ///       ^
 /// ```
 pub fn container_new_after(tokenizer: &mut Tokenizer) -> State {
-    // It was a new block quote or a list.
+    // It was a new block quote, list item, or footnote definition.
     // Swap the new container (in the middle) with the existing one (at the end).
     // Take the new container.
     let container = tokenizer
@@ -453,6 +479,7 @@ fn exit_containers(tokenizer: &mut Tokenizer, phase: &Phase) {
             let container = stack_close.pop().unwrap();
             let name = match container.kind {
                 Container::BlockQuote => Name::BlockQuote,
+                Container::GfmFootnoteDefinition => Name::GfmFootnoteDefinition,
                 Container::ListItem => Name::ListItem,
             };
 
diff --git a/src/construct/gfm_footnote_definition.rs b/src/construct/gfm_footnote_definition.rs
new file mode 100644
index 0000000..3715044
--- /dev/null
+++ b/src/construct/gfm_footnote_definition.rs
@@ -0,0 +1,345 @@
+//! GFM: Footnote definition occurs in the [document][] content type.
+//!
+//! ## Grammar
+//!
+//! Footnote definitions form with the following BNF
+//! (<small>see [construct][crate::construct] for character groups</small>):
+//!
+//! ```bnf
+//! ; Restriction: `label` must start with `^` (and not be empty after it).
+//! ; See the `label` construct for the BNF of that parts.
+//! gfm_footnote_definition_start ::= label ':' *space_or_tab
+//!
+//! ; Restriction: blank line allowed.
+//! gfm_footnote_definition_cont ::= 4(space_or_tab)
+//! ```
+//!
+//! Further lines that are not prefixed with `gfm_footnote_definition_cont`
+//! cause the footnote definition to be exited, except when those lines are
+//! lazy continuation or blank.
+//! Like so many things in markdown, footnote definition too, are complex.
+//! See [*§ Phase 1: block structure* in `CommonMark`][commonmark_block] for
+//! more on parsing details.
+//!
+//! See [`label`][label] for grammar, notes, and recommendations on that part.
+//!
+//! The `label` part is interpreted as the [string][] content type.
+//! That means that [character escapes][character_escape] and
+//! [character references][character_reference] are allowed.
+//!
+//! Definitions match to calls through identifiers.
+//! To match, both labels must be equal after normalizing with
+//! [`normalize_identifier`][normalize_identifier].
+//! One definition can match to multiple calls.
+//! Multiple definitions with the same, normalized, identifier are ignored: the
+//! first definition is preferred.
+//! To illustrate, the definition with the content of `x` wins:
+//!
+//! ```markdown
+//! [^a]: x
+//! [^a]: y
+//!
+//! [^a]
+//! ```
+//!
+//! Importantly, while labels *can* include [string][] content (character
+//! escapes and character references), these are not considered when matching.
+//! To illustrate, neither definition matches the call:
+//!
+//! ```markdown
+//! [^a&amp;b]: x
+//! [^a\&b]: y
+//!
+//! [^a&b]
+//! ```
+//!
+//! Because footnote definitions are containers (like block quotes and list
+//! items), they can contain more footnote definitions, and they can include
+//! calls to themselves.
+//!
+//! ## HTML
+//!
+//! GFM footnote definitions do not, on their own, relate to anything in HTML.
+//! When matched with a [label end][label_end], which in turns matches to a
+//! [GFM label start (footnote)][gfm_label_start_footnote], the definition
+//! relates to several elements in HTML.
+//!
+//! When one or more definitions are called, a footnote section is generated
+//! at the end of the document, using `<section>`, `<h2>`, and `<ol>` elements:
+//!
+//! ```html
+//! <section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+//! <ol>…</ol>
+//! </section>
+//! ```
+//!
+//! Each definition is generated as a `<li>` in the `<ol>`, in the order they
+//! were first called:
+//!
+//! ```html
+//! <li id="user-content-fn-1">…</li>
+//! ```
+//!
+//! Backreferences are injected at the end of the first paragraph, or, when
+//! there is no paragraph, at the end of the definition.
+//! When a definition is called multiple times, multiple backreferences are
+//! generated.
+//! Further backreferences use an extra counter in the `href` attribute and
+//! visually in a `<span>` after `↩`.
+//!
+//! ```html
+//! <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a> <a href="#user-content-fnref-1-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩<sup>2</sup></a>
+//! ```
+//!
+//! See
+//! [*§ 4.5.1 The `a` element*][html_a],
+//! [*§ 4.3.6 The `h1`, `h2`, `h3`, `h4`, `h5`, and `h6` elements*][html_h],
+//! [*§ 4.4.8 The `li` element*][html_li],
+//! [*§ 4.4.5 The `ol` element*][html_ol],
+//! [*§ 4.4.1 The `p` element*][html_p],
+//! [*§ 4.3.3 The `section` element*][html_section], and
+//! [*§ 4.5.19 The `sub` and `sup` elements*][html_sup]
+//! in the HTML spec for more info.
+//!
+//! ## Recommendation
+//!
+//! When authoring markdown with footnotes, it’s recommended to use words
+//! instead of numbers (or letters or anything with an order) as calls.
+//! That makes it easier to reuse and reorder footnotes.
+//!
+//! It’s recommended to place footnotes definitions at the bottom of the document.
+//!
+//! ## Bugs
+//!
+//! GitHub’s own algorithm to parse footnote definitions contains several bugs.
+//! These are not present in this project.
+//! The issues relating to footnote definitions are:
+//!
+//! *   [Footnote reference call identifiers are trimmed, but definition identifiers aren’t](https://github.com/github/cmark-gfm/issues/237)\
+//!     — initial and final whitespace in labels causes them not to match
+//! *   [Footnotes are matched case-insensitive, but links keep their casing, breaking them](https://github.com/github/cmark-gfm/issues/239)\
+//!     — using uppercase (or any character that will be percent encoded) in identifiers breaks links
+//! *   [Colons in footnotes generate links w/o `href`](https://github.com/github/cmark-gfm/issues/250)\
+//!     — colons in identifiers generate broken links
+//! *   [Character escape of `]` does not work in footnote identifiers](https://github.com/github/cmark-gfm/issues/240)\
+//!     — some character escapes don’t work
+//! *   [Footnotes in links are broken](https://github.com/github/cmark-gfm/issues/249)\
+//!     — while `CommonMark` prevents links in links, GitHub does not prevent footnotes (which turn into links) in links
+//! *   [Footnote-like brackets around image, break that image](https://github.com/github/cmark-gfm/issues/275)\
+//!     — images can’t be used in what looks like a footnote call
+//!
+//! ## Tokens
+//!
+//! *   [`DefinitionMarker`][Name::DefinitionMarker]
+//! *   [`GfmFootnoteDefinition`][Name::GfmFootnoteDefinition]
+//! *   [`GfmFootnoteDefinitionLabel`][Name::GfmFootnoteDefinitionLabel]
+//! *   [`GfmFootnoteDefinitionLabelMarker`][Name::GfmFootnoteDefinitionLabelMarker]
+//! *   [`GfmFootnoteDefinitionLabelString`][Name::GfmFootnoteDefinitionLabelString]
+//! *   [`GfmFootnoteDefinitionMarker`][Name::GfmFootnoteDefinitionMarker]
+//! *   [`GfmFootnoteDefinitionPrefix`][Name::GfmFootnoteDefinitionPrefix]
+//! *   [`SpaceOrTab`][Name::SpaceOrTab]
+//!
+//! ## References
+//!
+//! *   [`micromark-extension-gfm-task-list-item`](https://github.com/micromark/micromark-extension-gfm-footnote)
+//!
+//! > 👉 **Note**: Footnotes are not specified in GFM yet.
+//! > See [`github/cmark-gfm#270`](https://github.com/github/cmark-gfm/issues/270)
+//! > for the related issue.
+//!
+//! [document]: crate::construct::document
+//! [string]: crate::construct::string
+//! [character_reference]: crate::construct::character_reference
+//! [character_escape]: crate::construct::character_escape
+//! [label]: crate::construct::partial_label
+//! [label_end]: crate::construct::label_end
+//! [gfm_label_start_footnote]: crate::construct::gfm_label_start_footnote
+//! [commonmark_block]: https://spec.commonmark.org/0.30/#phase-1-block-structure
+//! [html_a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element
+//! [html_h]: https://html.spec.whatwg.org/multipage/sections.html#the-h1,-h2,-h3,-h4,-h5,-and-h6-elements
+//! [html_li]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-li-element
+//! [html_ol]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-ol-element
+//! [html_p]: https://html.spec.whatwg.org/multipage/grouping-content.html#the-p-element
+//! [html_section]: https://html.spec.whatwg.org/multipage/sections.html#the-section-element
+//! [html_sup]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-sub-and-sup-elements
+
+use crate::construct::partial_space_or_tab::space_or_tab_min_max;
+use crate::event::Name;
+use crate::state::{Name as StateName, State};
+use crate::tokenizer::Tokenizer;
+use crate::util::{
+    constant::TAB_SIZE,
+    normalize_identifier::normalize_identifier,
+    skip,
+    slice::{Position, Slice},
+};
+
+/// Start of GFM footnote definition.
+///
+/// ```markdown
+/// > | [^a]: b
+///     ^
+/// ```
+pub fn start(tokenizer: &mut Tokenizer) -> State {
+    if tokenizer
+        .parse_state
+        .options
+        .constructs
+        .gfm_footnote_definition
+    {
+        tokenizer.enter(Name::GfmFootnoteDefinition);
+
+        if matches!(tokenizer.current, Some(b'\t' | b' ')) {
+            tokenizer.attempt(
+                State::Next(StateName::GfmFootnoteDefinitionLabelBefore),
+                State::Nok,
+            );
+            State::Retry(space_or_tab_min_max(
+                tokenizer,
+                1,
+                if tokenizer.parse_state.options.constructs.code_indented {
+                    TAB_SIZE - 1
+                } else {
+                    usize::MAX
+                },
+            ))
+        } else {
+            State::Retry(StateName::GfmFootnoteDefinitionLabelBefore)
+        }
+    } else {
+        State::Nok
+    }
+}
+
+/// Before definition label (after optional whitespace).
+///
+/// ```markdown
+/// > | [^a]: b
+///     ^
+/// ```
+pub fn label_before(tokenizer: &mut Tokenizer) -> State {
+    match tokenizer.current {
+        Some(b'[') => {
+            tokenizer.tokenize_state.token_1 = Name::GfmFootnoteDefinitionLabel;
+            tokenizer.tokenize_state.token_2 = Name::GfmFootnoteDefinitionLabelMarker;
+            tokenizer.tokenize_state.token_3 = Name::GfmFootnoteDefinitionLabelString;
+            tokenizer.tokenize_state.token_4 = Name::GfmFootnoteDefinitionMarker;
+            tokenizer.tokenize_state.marker = b'^';
+            tokenizer.enter(Name::GfmFootnoteDefinitionPrefix);
+            tokenizer.attempt(
+                State::Next(StateName::GfmFootnoteDefinitionLabelAfter),
+                State::Nok,
+            );
+            State::Retry(StateName::LabelStart)
+        }
+        _ => State::Nok,
+    }
+}
+
+/// After definition label.
+///
+/// ```markdown
+/// > | [^a]: b
+///         ^
+/// ```
+pub fn label_after(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.tokenize_state.token_1 = Name::Data;
+    tokenizer.tokenize_state.token_2 = Name::Data;
+    tokenizer.tokenize_state.token_3 = Name::Data;
+    tokenizer.tokenize_state.token_4 = Name::Data;
+    tokenizer.tokenize_state.marker = 0;
+
+    match tokenizer.current {
+        Some(b':') => {
+            let end = skip::to_back(
+                &tokenizer.events,
+                tokenizer.events.len() - 1,
+                &[Name::GfmFootnoteDefinitionLabelString],
+            );
+
+            // Note: we don’t care about virtual spaces, so `as_str` is fine.
+            let id = normalize_identifier(
+                Slice::from_position(
+                    tokenizer.parse_state.bytes,
+                    &Position::from_exit_event(&tokenizer.events, end),
+                )
+                .as_str(),
+            );
+
+            // Note: we don’t care about uniqueness.
+            // It’s likely that that doesn’t happen very frequently.
+            // It is more likely that it wastes precious time.
+            tokenizer.tokenize_state.gfm_footnote_definitions.push(id);
+
+            tokenizer.enter(Name::DefinitionMarker);
+            tokenizer.consume();
+            tokenizer.exit(Name::DefinitionMarker);
+            tokenizer.attempt(
+                State::Next(StateName::GfmFootnoteDefinitionWhitespaceAfter),
+                State::Nok,
+            );
+            // Any whitespace after the marker is eaten, forming indented code
+            // is not possible.
+            // No space is also fine, just like a block quote marker.
+            State::Next(space_or_tab_min_max(tokenizer, 0, usize::MAX))
+        }
+        _ => State::Nok,
+    }
+}
+
+/// After definition prefix.
+///
+/// ```markdown
+/// > | [^a]: b
+///           ^
+/// ```
+pub fn whitespace_after(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.exit(Name::GfmFootnoteDefinitionPrefix);
+    State::Ok
+}
+
+/// Start of footnote definition continuation.
+///
+/// ```markdown
+///   | [^a]: b
+/// > |     c
+///     ^
+/// ```
+pub fn cont_start(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.check(
+        State::Next(StateName::GfmFootnoteDefinitionContBlank),
+        State::Next(StateName::GfmFootnoteDefinitionContFilled),
+    );
+    State::Retry(StateName::BlankLineStart)
+}
+
+/// Start of footnote definition continuation, at a blank line.
+///
+/// ```markdown
+///   | [^a]: b
+/// > | ␠␠␊
+///     ^
+/// ```
+pub fn cont_blank(tokenizer: &mut Tokenizer) -> State {
+    if matches!(tokenizer.current, Some(b'\t' | b' ')) {
+        State::Retry(space_or_tab_min_max(tokenizer, 0, TAB_SIZE))
+    } else {
+        State::Ok
+    }
+}
+
+/// Start of footnote definition continuation, at a filled line.
+///
+/// ```markdown
+///   | [^a]: b
+/// > |     c
+///     ^
+/// ```
+pub fn cont_filled(tokenizer: &mut Tokenizer) -> State {
+    if matches!(tokenizer.current, Some(b'\t' | b' ')) {
+        // Consume exactly `TAB_SIZE`.
+        State::Retry(space_or_tab_min_max(tokenizer, TAB_SIZE, TAB_SIZE))
+    } else {
+        State::Nok
+    }
+}
diff --git a/src/construct/gfm_label_start_footnote.rs b/src/construct/gfm_label_start_footnote.rs
new file mode 100644
index 0000000..a3a0df6
--- /dev/null
+++ b/src/construct/gfm_label_start_footnote.rs
@@ -0,0 +1,91 @@
+//! Label start (footnote) occurs in the [text][] content type.
+//!
+//! ## Grammar
+//!
+//! Label start (footnote) forms with the following BNF
+//! (<small>see [construct][crate::construct] for character groups</small>):
+//!
+//! ```bnf
+//! gfm_label_start_footnote ::= '[' '^'
+//! ```
+//!
+//! ## HTML
+//!
+//! Label start (footnote) does not, on its own, relate to anything in HTML.
+//! When matched with a [label end][label_end], they together relate to `<sup>`
+//! and `<a>` elements in HTML.
+//! See [*§ 4.5.19 The `sub` and `sup` elements*][html_sup] and
+//! [*§ 4.5.1 The `a` element*][html_a] in the HTML spec for more info.
+//! Without an end, the characters (`[^`) are output.
+//!
+//! ## Tokens
+//!
+//! *   [`LabelImage`][Name::LabelImage]
+//! *   To do.
+//!
+//! ## References
+//!
+//! *   [`micromark-extension-gfm-footnote`](https://github.com/micromark/micromark-extension-gfm-footnote)
+//!
+//! > 👉 **Note**: Footnotes are not specified in GFM yet.
+//! > See [`github/cmark-gfm#270`](https://github.com/github/cmark-gfm/issues/270)
+//! > for the related issue.
+//!
+//! [text]: crate::construct::text
+//! [label_end]: crate::construct::label_end
+//! [html_a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element
+//! [html_sup]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-sub-and-sup-elements
+
+use crate::event::Name;
+use crate::resolve::Name as ResolveName;
+use crate::state::{Name as StateName, State};
+use crate::tokenizer::{LabelKind, LabelStart, Tokenizer};
+
+/// Start of label (footnote) start.
+///
+/// ```markdown
+/// > | a [^b] c
+///       ^
+/// ```
+pub fn start(tokenizer: &mut Tokenizer) -> State {
+    if tokenizer
+        .parse_state
+        .options
+        .constructs
+        .gfm_label_start_footnote
+        && tokenizer.current == Some(b'[')
+    {
+        tokenizer.enter(Name::GfmFootnoteCallLabel);
+        tokenizer.enter(Name::LabelMarker);
+        tokenizer.consume();
+        tokenizer.exit(Name::LabelMarker);
+        State::Next(StateName::GfmLabelStartFootnoteOpen)
+    } else {
+        State::Nok
+    }
+}
+
+/// After `[`, at `^`.
+///
+/// ```markdown
+/// > | a [^b] c
+///        ^
+/// ```
+pub fn open(tokenizer: &mut Tokenizer) -> State {
+    match tokenizer.current {
+        Some(b'^') => {
+            tokenizer.enter(Name::GfmFootnoteCallMarker);
+            tokenizer.consume();
+            tokenizer.exit(Name::GfmFootnoteCallMarker);
+            tokenizer.exit(Name::GfmFootnoteCallLabel);
+            tokenizer.tokenize_state.label_starts.push(LabelStart {
+                kind: LabelKind::GfmFootnote,
+                start: (tokenizer.events.len() - 6, tokenizer.events.len() - 1),
+                inactive: false,
+            });
+            tokenizer.register_resolver_before(ResolveName::Label);
+            State::Ok
+        }
+        _ => State::Nok,
+    }
+}
diff --git a/src/construct/label_end.rs b/src/construct/label_end.rs
index 0ea745f..b5a6013 100644
--- a/src/construct/label_end.rs
+++ b/src/construct/label_end.rs
@@ -46,6 +46,8 @@
 //! attribute in case of a [label start (link)][label_start_link], and an
 //! `src` attribute in case of a [label start (image)][label_start_image].
 //! The title is formed, optionally, on either `<a>` or `<img>`.
+//! When matched with a [gfm label start (footnote)][gfm_label_start_footnote],
+//! no reference or resource can follow the label end.
 //!
 //! For info on how to encode characters in URLs, see
 //! [`destination`][destination].
@@ -53,11 +55,13 @@
 //! `<img>` when compiling, see
 //! [`sanitize_uri`][sanitize_uri].
 //!
+//! In case of a matched [gfm label start (footnote)][gfm_label_start_footnote],
+//! a counter is injected.
 //! In case of a matched [label start (link)][label_start_link], the interpreted
 //! content between it and the label end, is placed between the opening and
 //! closing tags.
-//! Otherwise, the text is also interpreted, but used *without* the resulting
-//! tags:
+//! In case of a matched [label start (image)][label_start_image], the text is
+//! also interpreted, but used *without* the resulting tags:
 //!
 //! ```markdown
 //! [a *b* c](#)
@@ -75,8 +79,9 @@
 //! It is possible to use images in links.
 //! It’s somewhat possible to have links in images (the text will be used, not
 //! the HTML, see above).
-//! But it’s not possible to use links in links.
-//! The “deepest” link wins.
+//! But it’s not possible to use links (or footnotes, which result in links)
+//! in links.
+//! The “deepest” link (or footnote) wins.
 //! To illustrate:
 //!
 //! ```markdown
@@ -104,17 +109,26 @@
 //! It can also match with [label start (image)][label_start_image], in which
 //! case they form an `<img>` element.
 //! See [*§ 4.8.3 The `img` element*][html_img] in the HTML spec for more info.
+//! It can also match with [gfm label start (footnote)][gfm_label_start_footnote],
+//! in which case they form `<sup>` and `<a>` elements in HTML.
+//! See [*§ 4.5.19 The `sub` and `sup` elements*][html_sup] and
+//! [*§ 4.5.1 The `a` element*][html_a] in the HTML spec for more info.
 //!
 //! ## Recommendation
 //!
-//! It is recommended to use labels instead of [autolinks][autolink].
+//! It is recommended to use labels for links instead of [autolinks][autolink].
 //! Labels allow more characters in URLs, and allow relative URLs and `www.`
 //! URLs.
 //! They also allow for descriptive text to explain the URL in prose.
 //!
+//! In footnotes, it’s recommended to use words instead of numbers (or letters
+//! or anything with an order) as calls.
+//! That makes it easier to reuse and reorder footnotes.
+//!
 //! ## Tokens
 //!
 //! *   [`Data`][Name::Data]
+//! *   [`GfmFootnoteCall`][Name::GfmFootnoteCall]
 //! *   [`Image`][Name::Image]
 //! *   [`Label`][Name::Label]
 //! *   [`LabelEnd`][Name::LabelEnd]
@@ -140,10 +154,15 @@
 //! ## References
 //!
 //! *   [`label-end.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/label-end.js)
+//! *   [`micromark-extension-gfm-task-list-item`](https://github.com/micromark/micromark-extension-gfm-footnote)
 //! *   [*§ 4.7 Link reference definitions* in `CommonMark`](https://spec.commonmark.org/0.30/#link-reference-definitions)
 //! *   [*§ 6.3 Links* in `CommonMark`](https://spec.commonmark.org/0.30/#links)
 //! *   [*§ 6.4 Images* in `CommonMark`](https://spec.commonmark.org/0.30/#images)
 //!
+//! > 👉 **Note**: Footnotes are not specified in GFM yet.
+//! > See [`github/cmark-gfm#270`](https://github.com/github/cmark-gfm/issues/270)
+//! > for the related issue.
+//!
 //! [string]: crate::construct::string
 //! [text]: crate::construct::text
 //! [destination]: crate::construct::partial_destination
@@ -151,25 +170,28 @@
 //! [label]: crate::construct::partial_label
 //! [label_start_image]: crate::construct::label_start_image
 //! [label_start_link]: crate::construct::label_start_link
+//! [gfm_label_start_footnote]: crate::construct::gfm_label_start_footnote
 //! [definition]: crate::construct::definition
 //! [autolink]: crate::construct::autolink
 //! [sanitize_uri]: crate::util::sanitize_uri::sanitize_uri
 //! [normalize_identifier]: crate::util::normalize_identifier::normalize_identifier
 //! [html_a]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-a-element
 //! [html_img]: https://html.spec.whatwg.org/multipage/embedded-content.html#the-img-element
+//! [html_sup]: https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-sub-and-sup-elements
 
 use crate::construct::partial_space_or_tab_eol::space_or_tab_eol;
 use crate::event::{Event, Kind, Name};
 use crate::resolve::Name as ResolveName;
 use crate::state::{Name as StateName, State};
-use crate::tokenizer::{Label, LabelStart, Tokenizer};
+use crate::tokenizer::{Label, LabelKind, LabelStart, Tokenizer};
 use crate::util::{
     constant::RESOURCE_DESTINATION_BALANCE_MAX,
     normalize_identifier::normalize_identifier,
     skip,
     slice::{Position, Slice},
 };
-use alloc::vec;
+use alloc::{string::String, vec};
+extern crate std;
 
 /// Start of label end.
 ///
@@ -190,7 +212,15 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
 
             tokenizer.tokenize_state.end = tokenizer.events.len();
 
-            // Mark as balanced if the info is inactive.
+            // If the corresponding label (link) start is marked as inactive,
+            // it means we’d be wrapping a link, like this:
+            //
+            // ```markdown
+            // > | a [b [c](d) e](f) g.
+            //                  ^
+            // ```
+            //
+            // We can’t have that, so it’s just balanced brackets.
             if label_start.inactive {
                 return State::Retry(StateName::LabelEndNok);
             }
@@ -220,19 +250,34 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
 ///       ^
 /// ```
 pub fn after(tokenizer: &mut Tokenizer) -> State {
-    let start = tokenizer.tokenize_state.label_starts.last().unwrap();
-    let defined = tokenizer
-        .parse_state
-        .definitions
-        .contains(&normalize_identifier(
-            // We don’t care about virtual spaces, so `indices` and `as_str` are fine.
-            Slice::from_indices(
-                tokenizer.parse_state.bytes,
-                tokenizer.events[start.start.1].point.index,
-                tokenizer.events[tokenizer.tokenize_state.end].point.index,
-            )
-            .as_str(),
-        ));
+    let start_index = tokenizer.tokenize_state.label_starts.len() - 1;
+    let start = &tokenizer.tokenize_state.label_starts[start_index];
+
+    let indices = (
+        tokenizer.events[start.start.1].point.index,
+        tokenizer.events[tokenizer.tokenize_state.end].point.index,
+    );
+
+    // We don’t care about virtual spaces, so `indices` and `as_str` are fine.
+    let mut id = normalize_identifier(
+        Slice::from_indices(tokenizer.parse_state.bytes, indices.0, indices.1).as_str(),
+    );
+
+    // See if this matches a footnote definition.
+    if start.kind == LabelKind::GfmFootnote {
+        if tokenizer.parse_state.gfm_footnote_definitions.contains(&id) {
+            return State::Retry(StateName::LabelEndOk);
+        }
+
+        // Nope, this might be a normal link?
+        tokenizer.tokenize_state.label_starts[start_index].kind = LabelKind::GfmUndefinedFootnote;
+        let mut new_id = String::new();
+        new_id.push('^');
+        new_id.push_str(&id);
+        id = new_id;
+    }
+
+    let defined = tokenizer.parse_state.definitions.contains(&id);
 
     match tokenizer.current {
         // Resource (`[asd](fgh)`)?
@@ -302,17 +347,15 @@ pub fn ok(tokenizer: &mut Tokenizer) -> State {
     // Remove the start.
     let label_start = tokenizer.tokenize_state.label_starts.pop().unwrap();
 
-    let is_link = tokenizer.events[label_start.start.0].name == Name::LabelLink;
-
-    // If this is a link, we need to mark earlier link starts as no longer
-    // viable for use (as they would otherwise contain a link).
+    // If this is a link or footnote, we need to mark earlier link starts as no
+    // longer viable for use (as they would otherwise contain a link).
     // These link starts are still looking for balanced closing brackets, so
-    // we can’t remove them.
-    if is_link {
+    // we can’t remove them, but we can mark them.
+    if label_start.kind != LabelKind::Image {
         let mut index = 0;
         while index < tokenizer.tokenize_state.label_starts.len() {
             let label_start = &mut tokenizer.tokenize_state.label_starts[index];
-            if tokenizer.events[label_start.start.0].name == Name::LabelLink {
+            if label_start.kind != LabelKind::Image {
                 label_start.inactive = true;
             }
             index += 1;
@@ -320,6 +363,7 @@ pub fn ok(tokenizer: &mut Tokenizer) -> State {
     }
 
     tokenizer.tokenize_state.labels.push(Label {
+        kind: label_start.kind,
         start: label_start.start,
         end: (tokenizer.tokenize_state.end, tokenizer.events.len() - 1),
     });
@@ -342,9 +386,7 @@ pub fn ok(tokenizer: &mut Tokenizer) -> State {
 /// ```
 pub fn nok(tokenizer: &mut Tokenizer) -> State {
     let start = tokenizer.tokenize_state.label_starts.pop().unwrap();
-
     tokenizer.tokenize_state.label_starts_loose.push(start);
-
     tokenizer.tokenize_state.end = 0;
     State::Nok
 }
@@ -615,120 +657,142 @@ pub fn reference_collapsed_open(tokenizer: &mut Tokenizer) -> State {
     }
 }
 
-/// Resolve media.
+/// Resolve images, links, and footnotes.
 ///
-/// This turns matching label start (image, link) and label ends into links and
-/// images, and turns unmatched label starts back into data.
+/// This turns matching label starts and label ends into links, images, and
+/// footnotes, and turns unmatched label starts back into data.
 pub fn resolve(tokenizer: &mut Tokenizer) {
-    let list = tokenizer.tokenize_state.label_starts.split_off(0);
-    mark_as_data(tokenizer, &list);
-    let list = tokenizer.tokenize_state.label_starts_loose.split_off(0);
-    mark_as_data(tokenizer, &list);
+    // Inject labels.
+    let labels = tokenizer.tokenize_state.labels.split_off(0);
+    inject_labels(tokenizer, &labels);
+    // Handle loose starts.
+    let starts = tokenizer.tokenize_state.label_starts.split_off(0);
+    mark_as_data(tokenizer, &starts);
+    let starts = tokenizer.tokenize_state.label_starts_loose.split_off(0);
+    mark_as_data(tokenizer, &starts);
 
-    let media = tokenizer.tokenize_state.labels.split_off(0);
+    tokenizer.map.consume(&mut tokenizer.events);
+}
 
+/// Inject links/images/footnotes.
+fn inject_labels(tokenizer: &mut Tokenizer, labels: &[Label]) {
     // Add grouping events.
     let mut index = 0;
-    while index < media.len() {
-        let media = &media[index];
-        // LabelLink:Enter or LabelImage:Enter.
-        let group_enter_index = media.start.0;
-        let group_enter_event = &tokenizer.events[group_enter_index];
-        // LabelLink:Exit or LabelImage:Exit.
-        let text_enter_index = media.start.0
-            + (if group_enter_event.name == Name::LabelLink {
-                4
-            } else {
-                6
-            });
-        // LabelEnd:Enter.
-        let text_exit_index = media.end.0;
-        // LabelEnd:Exit.
-        let label_exit_index = media.end.0 + 3;
-        // Resource:Exit, etc.
-        let group_end_index = media.end.1;
-
-        let group_name = if group_enter_event.name == Name::LabelLink {
-            Name::Link
-        } else {
+    while index < labels.len() {
+        let label = &labels[index];
+        let group_name = if label.kind == LabelKind::GfmFootnote {
+            Name::GfmFootnoteCall
+        } else if label.kind == LabelKind::Image {
             Name::Image
+        } else {
+            Name::Link
         };
 
+        // If this is a fine link, which starts with a footnote start that did
+        // not match, we need to inject the caret as data.
+        let mut caret = vec![];
+
+        if label.kind == LabelKind::GfmUndefinedFootnote {
+            // Add caret.
+            caret.push(Event {
+                kind: Kind::Enter,
+                name: Name::Data,
+                // Enter:GfmFootnoteCallMarker.
+                point: tokenizer.events[label.start.1 - 2].point.clone().clone(),
+                link: None,
+            });
+            caret.push(Event {
+                kind: Kind::Exit,
+                name: Name::Data,
+                // Exit:GfmFootnoteCallMarker.
+                point: tokenizer.events[label.start.1 - 1].point.clone(),
+                link: None,
+            });
+            // Change and move label end.
+            tokenizer.events[label.start.0].name = Name::LabelLink;
+            tokenizer.events[label.start.1].name = Name::LabelLink;
+            tokenizer.events[label.start.1].point = caret[0].point.clone();
+            // Remove the caret.
+            // Enter:GfmFootnoteCallMarker, Exit:GfmFootnoteCallMarker.
+            tokenizer.map.add(label.start.1 - 2, 2, vec![]);
+        }
+
         // Insert a group enter and label enter.
         tokenizer.map.add(
-            group_enter_index,
+            label.start.0,
             0,
             vec![
                 Event {
                     kind: Kind::Enter,
                     name: group_name.clone(),
-                    point: group_enter_event.point.clone(),
+                    point: tokenizer.events[label.start.0].point.clone(),
                     link: None,
                 },
                 Event {
                     kind: Kind::Enter,
                     name: Name::Label,
-                    point: group_enter_event.point.clone(),
+                    point: tokenizer.events[label.start.0].point.clone(),
                     link: None,
                 },
             ],
         );
 
         // Empty events not allowed.
-        if text_enter_index != text_exit_index {
-            // Insert a text enter.
+        // Though: if this was what looked like a footnote, but didn’t match,
+        // it’s a link instead, and we need to inject the `^`.
+        if label.start.1 != label.end.0 || !caret.is_empty() {
             tokenizer.map.add(
-                text_enter_index,
+                label.start.1 + 1,
                 0,
                 vec![Event {
                     kind: Kind::Enter,
                     name: Name::LabelText,
-                    point: tokenizer.events[text_enter_index].point.clone(),
+                    point: tokenizer.events[label.start.1].point.clone(),
                     link: None,
                 }],
             );
-
-            // Insert a text exit.
             tokenizer.map.add(
-                text_exit_index,
+                label.end.0,
                 0,
                 vec![Event {
                     kind: Kind::Exit,
                     name: Name::LabelText,
-                    point: tokenizer.events[text_exit_index].point.clone(),
+                    point: tokenizer.events[label.end.0].point.clone(),
                     link: None,
                 }],
             );
         }
 
+        if !caret.is_empty() {
+            tokenizer.map.add(label.start.1 + 1, 0, caret);
+        }
+
         // Insert a label exit.
         tokenizer.map.add(
-            label_exit_index + 1,
+            label.end.0 + 4,
             0,
             vec![Event {
                 kind: Kind::Exit,
                 name: Name::Label,
-                point: tokenizer.events[label_exit_index].point.clone(),
+                point: tokenizer.events[label.end.0 + 3].point.clone(),
                 link: None,
             }],
         );
 
         // Insert a group exit.
         tokenizer.map.add(
-            group_end_index + 1,
+            label.end.1 + 1,
             0,
             vec![Event {
                 kind: Kind::Exit,
                 name: group_name,
-                point: tokenizer.events[group_end_index].point.clone(),
+                point: tokenizer.events[label.end.1].point.clone(),
                 link: None,
             }],
         );
 
         index += 1;
     }
-
-    tokenizer.map.consume(&mut tokenizer.events);
 }
 
 /// Remove loose label starts.
diff --git a/src/construct/label_start_image.rs b/src/construct/label_start_image.rs
index a8c9ac3..4511794 100644
--- a/src/construct/label_start_image.rs
+++ b/src/construct/label_start_image.rs
@@ -35,7 +35,7 @@
 use crate::event::Name;
 use crate::resolve::Name as ResolveName;
 use crate::state::{Name as StateName, State};
-use crate::tokenizer::{LabelStart, Tokenizer};
+use crate::tokenizer::{LabelKind, LabelStart, Tokenizer};
 
 /// Start of label (image) start.
 ///
@@ -68,14 +68,52 @@ pub fn open(tokenizer: &mut Tokenizer) -> State {
             tokenizer.enter(Name::LabelMarker);
             tokenizer.consume();
             tokenizer.exit(Name::LabelMarker);
-            tokenizer.exit(Name::LabelImage);
-            tokenizer.tokenize_state.label_starts.push(LabelStart {
-                start: (tokenizer.events.len() - 6, tokenizer.events.len() - 1),
-                inactive: false,
-            });
-            tokenizer.register_resolver_before(ResolveName::Label);
-            State::Ok
+            State::Next(StateName::LabelStartImageAfter)
         }
         _ => State::Nok,
     }
 }
+
+/// After `![`.
+///
+/// ```markdown
+/// > | a ![b] c
+///         ^
+/// ```
+///
+/// This is needed in because, when GFM footnotes are enabled, images never
+/// form when started with a `^`.
+/// Instead, links form:
+///
+/// ```markdown
+/// ![^a](b)
+///
+/// ![^a][b]
+///
+/// [b]: c
+/// ```
+///
+/// ```html
+/// <p>!<a href=\"b\">^a</a></p>
+/// <p>!<a href=\"c\">^a</a></p>
+/// ```
+pub fn after(tokenizer: &mut Tokenizer) -> State {
+    if tokenizer
+        .parse_state
+        .options
+        .constructs
+        .gfm_label_start_footnote
+        && tokenizer.current == Some(b'^')
+    {
+        State::Nok
+    } else {
+        tokenizer.exit(Name::LabelImage);
+        tokenizer.tokenize_state.label_starts.push(LabelStart {
+            kind: LabelKind::Image,
+            start: (tokenizer.events.len() - 6, tokenizer.events.len() - 1),
+            inactive: false,
+        });
+        tokenizer.register_resolver_before(ResolveName::Label);
+        State::Ok
+    }
+}
diff --git a/src/construct/label_start_link.rs b/src/construct/label_start_link.rs
index 3aeb68b..3454724 100644
--- a/src/construct/label_start_link.rs
+++ b/src/construct/label_start_link.rs
@@ -34,7 +34,7 @@
 use crate::event::Name;
 use crate::resolve::Name as ResolveName;
 use crate::state::State;
-use crate::tokenizer::{LabelStart, Tokenizer};
+use crate::tokenizer::{LabelKind, LabelStart, Tokenizer};
 
 /// Start of label (link) start.
 ///
@@ -52,6 +52,7 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
         tokenizer.exit(Name::LabelMarker);
         tokenizer.exit(Name::LabelLink);
         tokenizer.tokenize_state.label_starts.push(LabelStart {
+            kind: LabelKind::Link,
             start: (start, tokenizer.events.len() - 1),
             inactive: false,
         });
diff --git a/src/construct/list_item.rs b/src/construct/list_item.rs
index 39b5d13..658c2c7 100644
--- a/src/construct/list_item.rs
+++ b/src/construct/list_item.rs
@@ -17,7 +17,7 @@
 //! ```
 //!
 //! Further lines that are not prefixed with `list_item_cont` cause the list
-//! item to be exited, except when those lines are lazy continuation.
+//! item to be exited, except when those lines are lazy continuation or blank.
 //! Like so many things in markdown, list items too, are complex.
 //! See [*§ Phase 1: block structure* in `CommonMark`][commonmark_block] for
 //! more on parsing details.
diff --git a/src/construct/mod.rs b/src/construct/mod.rs
index 7ac3899..c5002bb 100644
--- a/src/construct/mod.rs
+++ b/src/construct/mod.rs
@@ -59,6 +59,9 @@
 //!
 //! *   [frontmatter][]
 //! *   [gfm autolink literal][gfm_autolink_literal]
+//! *   [gfm footnote definition][gfm_footnote_definition]
+//! *   [gfm task list item check][gfm_task_list_item_check]
+//! *   [gfm label start footnote][gfm_label_start_footnote]
 //!
 //! There are also several small subroutines typically used in different places:
 //!
@@ -146,6 +149,8 @@ pub mod document;
 pub mod flow;
 pub mod frontmatter;
 pub mod gfm_autolink_literal;
+pub mod gfm_footnote_definition;
+pub mod gfm_label_start_footnote;
 pub mod gfm_task_list_item_check;
 pub mod hard_break_escape;
 pub mod heading_atx;
diff --git a/src/construct/partial_label.rs b/src/construct/partial_label.rs
index 47ffd90..ab436b2 100644
--- a/src/construct/partial_label.rs
+++ b/src/construct/partial_label.rs
@@ -81,13 +81,37 @@ pub fn start(tokenizer: &mut Tokenizer) -> State {
             tokenizer.enter(tokenizer.tokenize_state.token_2.clone());
             tokenizer.consume();
             tokenizer.exit(tokenizer.tokenize_state.token_2.clone());
-            tokenizer.enter(tokenizer.tokenize_state.token_3.clone());
-            State::Next(StateName::LabelAtBreak)
+            State::Next(StateName::LabelAtMarker)
         }
         _ => State::Nok,
     }
 }
 
+/// At an optional extra marker.
+///
+/// Used for footnotes.
+///
+/// ```markdown
+/// > | [^a]
+///      ^
+/// ```
+pub fn at_marker(tokenizer: &mut Tokenizer) -> State {
+    // For footnotes (and potentially other custom things in the future),
+    // We need to make sure there is a certain marker after `[`.
+    if tokenizer.tokenize_state.marker == 0 {
+        tokenizer.enter(tokenizer.tokenize_state.token_3.clone());
+        State::Retry(StateName::LabelAtBreak)
+    } else if tokenizer.current == Some(tokenizer.tokenize_state.marker) {
+        tokenizer.enter(tokenizer.tokenize_state.token_4.clone());
+        tokenizer.consume();
+        tokenizer.exit(tokenizer.tokenize_state.token_4.clone());
+        tokenizer.enter(tokenizer.tokenize_state.token_3.clone());
+        State::Next(StateName::LabelAtBreak)
+    } else {
+        State::Nok
+    }
+}
+
 /// In label, at something, before something else.
 ///
 /// ```markdown
diff --git a/src/construct/text.rs b/src/construct/text.rs
index 65f55d4..5535e3f 100644
--- a/src/construct/text.rs
+++ b/src/construct/text.rs
@@ -11,6 +11,8 @@
 //! *   [Character escape][crate::construct::character_escape]
 //! *   [Character reference][crate::construct::character_reference]
 //! *   [Code (text)][crate::construct::code_text]
+//! *   [GFM: Label start (footnote)][crate::construct::gfm_label_start_footnote]
+//! *   [GFM: Task list item check][crate::construct::gfm_task_list_item_check]
 //! *   [Hard break (escape)][crate::construct::hard_break_escape]
 //! *   [HTML (text)][crate::construct::html_text]
 //! *   [Label start (image)][crate::construct::label_start_image]
@@ -34,7 +36,7 @@ const MARKERS: [u8; 10] = [
     b'<',  // `autolink`, `html_text`
     b'[',  // `label_start_link`
     b'\\', // `character_escape`, `hard_break_escape`
-    b']',  // `label_end`
+    b']',  // `label_end`, `gfm_label_start_footnote`
     b'_',  // `attention`
     b'`',  // `code_text`
     b'~',  // `attention` (w/ `gfm_strikethrough`)
@@ -104,9 +106,9 @@ pub fn before(tokenizer: &mut Tokenizer) -> State {
         Some(b'[') => {
             tokenizer.attempt(
                 State::Next(StateName::TextBefore),
-                State::Next(StateName::TextBeforeData),
+                State::Next(StateName::TextBeforeLabelStartLink),
             );
-            State::Retry(StateName::LabelStartLinkStart)
+            State::Retry(StateName::GfmLabelStartFootnoteStart)
         }
         Some(b'\\') => {
             tokenizer.attempt(
@@ -165,6 +167,22 @@ pub fn before_hard_break_escape(tokenizer: &mut Tokenizer) -> State {
     State::Retry(StateName::HardBreakEscapeStart)
 }
 
+/// Before label start (link).
+///
+/// At `[`, which wasn’t a GFM label start (footnote).
+///
+/// ```markdown
+/// > | [a](b)
+///     ^
+/// ```
+pub fn before_label_start_link(tokenizer: &mut Tokenizer) -> State {
+    tokenizer.attempt(
+        State::Next(StateName::TextBefore),
+        State::Next(StateName::TextBeforeData),
+    );
+    State::Retry(StateName::LabelStartLinkStart)
+}
+
 /// Before data.
 ///
 /// ```markdown
diff --git a/src/event.rs b/src/event.rs
index f20c599..3b805e5 100644
--- a/src/event.rs
+++ b/src/event.rs
@@ -753,7 +753,8 @@ pub enum Name {
     /// ## Info
     ///
     /// *   **Context**:
-    ///     [`Definition`][Name::Definition]
+    ///     [`Definition`][Name::Definition],
+    ///     [`GfmFootnoteDefinition`][Name::GfmFootnoteDefinition]
     /// *   **Content model**:
     ///     void
     /// *   **Construct**:
@@ -1019,7 +1020,172 @@ pub enum Name {
     ///     ^^^^^^^^^^^^^^^
     /// ```
     GfmAutolinkLiteralWww,
-    /// GFM: Strikethrough.
+    /// GFM extension: whole footnote call.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [text content][crate::construct::text]
+    /// *   **Content model**:
+    ///     [`Label`][Name::Label]
+    /// *   **Construct**:
+    ///     [`label_end`][crate::construct::label_end]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | a [^b] c
+    ///       ^^^^
+    /// ```
+    GfmFootnoteCall,
+    /// GFM extension: label start (footnote).
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`Label`][Name::Label]
+    /// *   **Content model**:
+    ///     [`GfmFootnoteCallMarker`][Name::GfmFootnoteCallMarker],
+    ///     [`LabelMarker`][Name::LabelMarker]
+    /// *   **Construct**:
+    ///     [`gfm_label_start_footnote`][crate::construct::gfm_label_start_footnote]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | a [^b] c
+    ///       ^^
+    /// ```
+    GfmFootnoteCallLabel,
+    /// GFM extension: label start (footnote) marker.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteCallLabel`][Name::GfmFootnoteCallLabel]
+    /// *   **Content model**:
+    ///     void
+    /// *   **Construct**:
+    ///     [`gfm_label_start_footnote`][crate::construct::gfm_label_start_footnote]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | a [^b] c
+    ///        ^
+    /// ```
+    GfmFootnoteCallMarker,
+    /// GFM extension: whole footnote definition.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [document content][crate::construct::document]
+    /// *   **Content model**:
+    ///     [`GfmFootnoteDefinitionPrefix`][Name::GfmFootnoteDefinitionPrefix],
+    ///     [document content][crate::construct::flow]
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///     ^^^^^^^
+    /// ```
+    GfmFootnoteDefinition,
+    /// GFM extension: footnote definition prefix.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteDefinition`][Name::GfmFootnoteDefinition]
+    /// *   **Content model**:
+    ///     [`DefinitionMarker`][Name::DefinitionMarker],
+    ///     [`GfmFootnoteDefinitionLabel`][Name::GfmFootnoteDefinitionLabel],
+    ///     [`SpaceOrTab`][Name::SpaceOrTab]
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///     ^^^^^^
+    /// ```
+    GfmFootnoteDefinitionPrefix,
+    /// GFM extension: footnote definition label.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteDefinitionPrefix`][Name::GfmFootnoteDefinitionPrefix]
+    /// *   **Content model**:
+    ///     [`GfmFootnoteDefinitionLabelMarker`][Name::GfmFootnoteDefinitionLabelMarker],
+    ///     [`GfmFootnoteDefinitionLabelString`][Name::GfmFootnoteDefinitionLabelString],
+    ///     [`GfmFootnoteDefinitionMarker`][Name::GfmFootnoteDefinitionMarker]
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///     ^^^^
+    /// ```
+    GfmFootnoteDefinitionLabel,
+    /// GFM extension: footnote definition label marker.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteDefinitionLabel`][Name::GfmFootnoteDefinitionLabel]
+    /// *   **Content model**:
+    ///     void
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///     ^  ^
+    GfmFootnoteDefinitionLabelMarker,
+    /// GFM extension: footnote definition label string.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteDefinitionLabel`][Name::GfmFootnoteDefinitionLabel]
+    /// *   **Content model**:
+    ///     [string content][crate::construct::string]
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///       ^
+    GfmFootnoteDefinitionLabelString,
+    /// GFM extension: footnote definition marker.
+    ///
+    /// ## Info
+    ///
+    /// *   **Context**:
+    ///     [`GfmFootnoteDefinitionLabel`][Name::GfmFootnoteDefinitionLabel]
+    /// *   **Content model**:
+    ///     void
+    /// *   **Construct**:
+    ///     [`gfm_footnote_definition`][crate::construct::gfm_footnote_definition]
+    ///
+    /// ## Example
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///      ^
+    GfmFootnoteDefinitionMarker,
+    /// GFM extension: Strikethrough.
     ///
     /// ## Info
     ///
@@ -1038,7 +1204,7 @@ pub enum Name {
     ///     ^^^
     /// ```
     GfmStrikethrough,
-    /// Gfm: Strikethrough sequence.
+    /// GFM extension: Strikethrough sequence.
     ///
     /// ## Info
     ///
@@ -1056,7 +1222,7 @@ pub enum Name {
     ///     ^ ^
     /// ```
     GfmStrikethroughSequence,
-    /// Gfm: Strikethrough text.
+    /// GFM extension: Strikethrough text.
     ///
     /// ## Info
     ///
@@ -1074,7 +1240,7 @@ pub enum Name {
     ///      ^
     /// ```
     GfmStrikethroughText,
-    /// GFM: Task list item check.
+    /// GFM extension: task list item check.
     ///
     /// ## Info
     ///
@@ -1094,7 +1260,7 @@ pub enum Name {
     ///       ^^^
     /// ```
     GfmTaskListItemCheck,
-    /// GFM: Task list item check marker.
+    /// GFM extension: task list item check marker.
     ///
     /// ## Info
     ///
@@ -1112,7 +1278,7 @@ pub enum Name {
     ///       ^ ^
     /// ```
     GfmTaskListItemMarker,
-    /// GFM: Task list item value: checked.
+    /// GFM extension: task list item value: checked.
     ///
     /// ## Info
     ///
@@ -1130,7 +1296,7 @@ pub enum Name {
     ///        ^
     /// ```
     GfmTaskListItemValueChecked,
-    /// GFM: Task list item value: unchecked.
+    /// GFM extension: task list item value: unchecked.
     ///
     /// ## Info
     ///
@@ -2105,7 +2271,7 @@ pub enum Name {
 }
 
 /// List of void events, used to make sure everything is working well.
-pub const VOID_EVENTS: [Name; 50] = [
+pub const VOID_EVENTS: [Name; 53] = [
     Name::AttentionSequence,
     Name::AutolinkEmail,
     Name::AutolinkMarker,
@@ -2134,6 +2300,9 @@ pub const VOID_EVENTS: [Name; 50] = [
     Name::GfmAutolinkLiteralEmail,
     Name::GfmAutolinkLiteralProtocol,
     Name::GfmAutolinkLiteralWww,
+    Name::GfmFootnoteCallMarker,
+    Name::GfmFootnoteDefinitionLabelMarker,
+    Name::GfmFootnoteDefinitionMarker,
     Name::GfmStrikethroughSequence,
     Name::GfmTaskListItemMarker,
     Name::GfmTaskListItemValueChecked,
diff --git a/src/lib.rs b/src/lib.rs
index 5b7836c..fd5e500 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -171,7 +171,20 @@ pub struct Constructs {
     ///     ^^^^^^^^^^^^^^^^^^^
     /// ```
     pub gfm_autolink_literal: bool,
-    /// GFM: strikethrough.
+    /// GFM: footnote definition.
+    ///
+    /// ```markdown
+    /// > | [^a]: b
+    ///     ^^^^^^^
+    /// ```
+    pub gfm_footnote_definition: bool,
+    /// GFM: footnote label start.
+    ///
+    /// ```markdown
+    /// > | a[^b]
+    ///      ^^
+    /// ```
+    pub gfm_label_start_footnote: bool,
     ///
     /// ```markdown
     /// > | a ~b~ c.
@@ -283,6 +296,8 @@ impl Default for Constructs {
             definition: true,
             frontmatter: false,
             gfm_autolink_literal: false,
+            gfm_label_start_footnote: false,
+            gfm_footnote_definition: false,
             gfm_strikethrough: false,
             gfm_task_list_item: false,
             hard_break_escape: true,
@@ -308,6 +323,8 @@ impl Constructs {
     pub fn gfm() -> Self {
         Self {
             gfm_autolink_literal: true,
+            gfm_footnote_definition: true,
+            gfm_label_start_footnote: true,
             gfm_strikethrough: true,
             gfm_task_list_item: true,
             ..Self::default()
@@ -376,6 +393,206 @@ pub struct Options {
     /// ```
     pub allow_dangerous_protocol: bool,
 
+    /// Label to use for the footnotes section.
+    ///
+    /// Change it when the markdown is not in English.
+    /// Typically affects screen readers (change `gfm_footnote_label_attributes`
+    /// to make it visible).
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // `"Footnotes"` is used by default:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    ///
+    /// // Pass `gfm_footnote_label` to use something else:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_footnote_label: Some("Notes de bas de page".to_string()),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Notes de bas de page</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    /// ```
+    pub gfm_footnote_label: Option<String>,
+
+    /// HTML tag to use for the footnote label.
+    ///
+    /// Change it to match your document structure and play well with your CSS.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // `"h2"` is used by default:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    ///
+    /// // Pass `gfm_footnote_label_tag_name` to use something else:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_footnote_label_tag_name: Some("h1".to_string()),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h1 id=\"footnote-label\" class=\"sr-only\">Footnotes</h1>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    /// ```
+    pub gfm_footnote_label_tag_name: Option<String>,
+
+    /// Attributes to use on the footnote label.
+    ///
+    /// > 👉 **Note**: `id="footnote-label"` is always added, because footnote
+    /// > calls use it with `aria-describedby` to provide an accessible label.
+    ///
+    /// A `class="sr-only"` is added by default to hide the label from sighted
+    /// users.
+    /// Change it to make the label visible, or add other classes or other
+    /// attributes.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // `"class=\"sr-only\""` is used by default:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    ///
+    /// // Pass `gfm_footnote_label_attributes` to use something else:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_footnote_label_attributes: Some("class=\"footnote-heading\"".to_string()),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"footnote-heading\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    /// ```
+    pub gfm_footnote_label_attributes: Option<String>,
+
+    /// Label to use from backreferences back to their footnote call.
+    ///
+    /// Change it when the markdown is not in English.
+    /// Affects screen readers.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // `"Back to content"` is used by default:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    ///
+    /// // Pass `gfm_footnote_back_label` to use something else:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_footnote_back_label: Some("Arrière".to_string()),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Arrière\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    /// ```
+    pub gfm_footnote_back_label: Option<String>,
+
+    /// Prefix to use before the `id` attribute on footnotes to prevent them
+    /// from *clobbering*.
+    ///
+    /// DOM clobbering is this:
+    ///
+    /// ```html
+    /// <p id=x></p>
+    /// <script>alert(x) // `x` now refers to the DOM `p#x` element</script>
+    /// ```
+    ///
+    /// The above example shows that elements are made available by browsers,
+    /// by their ID, on the `window` object, which is a security risk because
+    /// you might be expecting some other variable at that place.
+    /// Using a prefix solves this problem.
+    ///
+    /// ## Examples
+    ///
+    /// ```
+    /// use micromark::{micromark, micromark_with_options, Options, Constructs};
+    ///
+    /// // `"user-content-"` is used by default:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"user-content-fn-a\">\n<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    ///
+    /// // Pass `gfm_footnote_clobber_prefix` to use something else:
+    /// assert_eq!(
+    ///     micromark_with_options(
+    ///         "[^a]\n\n[^a]: b",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_footnote_clobber_prefix: Some("".to_string()),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><sup><a href=\"#fn-a\" id=\"fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>\n<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>\n<ol>\n<li id=\"fn-a\">\n<p>b <a href=\"#fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>\n</li>\n</ol>\n</section>\n"
+    /// );
+    /// ```
+    pub gfm_footnote_clobber_prefix: Option<String>,
+
     /// Whether to support GFM strikethrough (if enabled in `constructs`) with
     /// a single tilde (default: true).
     ///
@@ -389,26 +606,26 @@ pub struct Options {
     /// // micromark supports single tildes by default:
     /// assert_eq!(
     ///     micromark_with_options(
-    ///       "~a~",
-    ///       &Options {
-    ///         constructs: Constructs::gfm(),
-    ///         ..Options::default()
-    ///       }
-    ///   ),
-    ///   "<p><del>a</del></p>"
+    ///         "~a~",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p><del>a</del></p>"
     /// );
     ///
     /// // Pass `gfm_strikethrough_single_tilde: false` to turn that off:
     /// assert_eq!(
     ///     micromark_with_options(
-    ///       "~a~",
-    ///       &Options {
-    ///         constructs: Constructs::gfm(),
-    ///         gfm_strikethrough_single_tilde: false,
-    ///         ..Options::default()
-    ///       }
-    ///   ),
-    ///   "<p>~a~</p>"
+    ///         "~a~",
+    ///         &Options {
+    ///             constructs: Constructs::gfm(),
+    ///             gfm_strikethrough_single_tilde: false,
+    ///             ..Options::default()
+    ///         }
+    ///     ),
+    ///     "<p>~a~</p>"
     /// );
     /// ```
     pub gfm_strikethrough_single_tilde: bool,
@@ -488,6 +705,11 @@ impl Default for Options {
         Self {
             allow_dangerous_html: false,
             allow_dangerous_protocol: false,
+            gfm_footnote_label: None,
+            gfm_footnote_label_tag_name: None,
+            gfm_footnote_label_attributes: None,
+            gfm_footnote_back_label: None,
+            gfm_footnote_clobber_prefix: None,
             gfm_strikethrough_single_tilde: true,
             default_line_ending: LineEnding::default(),
             constructs: Constructs::default(),
diff --git a/src/parser.rs b/src/parser.rs
index afa08ac..62b3e03 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -17,8 +17,10 @@ pub struct ParseState<'a> {
     pub options: &'a Options,
     /// List of chars.
     pub bytes: &'a [u8],
-    /// Set of defined identifiers.
+    /// Set of defined definition identifiers.
     pub definitions: Vec<String>,
+    /// Set of defined GFM footnote definition identifiers.
+    pub gfm_footnote_definitions: Vec<String>,
 }
 
 /// Turn a string of markdown into events.
@@ -29,6 +31,7 @@ pub fn parse<'a>(value: &'a str, options: &'a Options) -> (Vec<Event>, &'a [u8])
         options,
         bytes: value.as_bytes(),
         definitions: vec![],
+        gfm_footnote_definitions: vec![],
     };
 
     let mut tokenizer = Tokenizer::new(
@@ -50,7 +53,10 @@ pub fn parse<'a>(value: &'a str, options: &'a Options) -> (Vec<Event>, &'a [u8])
 
     let mut events = tokenizer.events;
 
-    parse_state.definitions = tokenizer.tokenize_state.definitions;
+    let footnote = tokenizer.tokenize_state.gfm_footnote_definitions;
+    let normal = tokenizer.tokenize_state.definitions;
+    parse_state.gfm_footnote_definitions = footnote;
+    parse_state.definitions = normal;
 
     while !subtokenize(&mut events, &parse_state) {}
 
diff --git a/src/state.rs b/src/state.rs
index 65ffbeb..6c3f563 100644
--- a/src/state.rs
+++ b/src/state.rs
@@ -116,6 +116,7 @@ pub enum Name {
     DocumentContainerNewBefore,
     DocumentContainerNewBeforeNotBlockQuote,
     DocumentContainerNewBeforeNotList,
+    DocumentContainerNewBeforeNotGfmFootnoteDefinition,
     DocumentContainerNewAfter,
     DocumentContainersAfter,
     DocumentFlowInside,
@@ -145,6 +146,17 @@ pub enum Name {
     FrontmatterCloseSequence,
     FrontmatterCloseAfter,
 
+    GfmFootnoteDefinitionStart,
+    GfmFootnoteDefinitionLabelBefore,
+    GfmFootnoteDefinitionLabelAfter,
+    GfmFootnoteDefinitionWhitespaceAfter,
+    GfmFootnoteDefinitionContStart,
+    GfmFootnoteDefinitionContBlank,
+    GfmFootnoteDefinitionContFilled,
+
+    GfmLabelStartFootnoteStart,
+    GfmLabelStartFootnoteOpen,
+
     GfmTaskListItemCheckStart,
     GfmTaskListItemCheckInside,
     GfmTaskListItemCheckClose,
@@ -230,6 +242,7 @@ pub enum Name {
     HtmlTextLineEndingAfterPrefix,
 
     LabelStart,
+    LabelAtMarker,
     LabelAtBreak,
     LabelEolAfter,
     LabelAtBlankLine,
@@ -256,6 +269,7 @@ pub enum Name {
 
     LabelStartImageStart,
     LabelStartImageOpen,
+    LabelStartImageAfter,
 
     LabelStartLinkStart,
 
@@ -299,6 +313,7 @@ pub enum Name {
     TextBefore,
     TextBeforeHtml,
     TextBeforeHardBreakEscape,
+    TextBeforeLabelStartLink,
     TextBeforeData,
 
     ThematicBreakStart,
@@ -421,6 +436,9 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
         Name::DocumentContainerNewBeforeNotList => {
             construct::document::container_new_before_not_list
         }
+        Name::DocumentContainerNewBeforeNotGfmFootnoteDefinition => {
+            construct::document::container_new_before_not_footnote_definition
+        }
         Name::DocumentContainerNewAfter => construct::document::container_new_after,
         Name::DocumentContainersAfter => construct::document::containers_after,
         Name::DocumentFlowEnd => construct::document::flow_end,
@@ -450,6 +468,19 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
         Name::FrontmatterCloseSequence => construct::frontmatter::close_sequence,
         Name::FrontmatterCloseAfter => construct::frontmatter::close_after,
 
+        Name::GfmFootnoteDefinitionStart => construct::gfm_footnote_definition::start,
+        Name::GfmFootnoteDefinitionLabelBefore => construct::gfm_footnote_definition::label_before,
+        Name::GfmFootnoteDefinitionLabelAfter => construct::gfm_footnote_definition::label_after,
+        Name::GfmFootnoteDefinitionWhitespaceAfter => {
+            construct::gfm_footnote_definition::whitespace_after
+        }
+        Name::GfmFootnoteDefinitionContStart => construct::gfm_footnote_definition::cont_start,
+        Name::GfmFootnoteDefinitionContBlank => construct::gfm_footnote_definition::cont_blank,
+        Name::GfmFootnoteDefinitionContFilled => construct::gfm_footnote_definition::cont_filled,
+
+        Name::GfmLabelStartFootnoteStart => construct::gfm_label_start_footnote::start,
+        Name::GfmLabelStartFootnoteOpen => construct::gfm_label_start_footnote::open,
+
         Name::GfmTaskListItemCheckStart => construct::gfm_task_list_item_check::start,
         Name::GfmTaskListItemCheckInside => construct::gfm_task_list_item_check::inside,
         Name::GfmTaskListItemCheckClose => construct::gfm_task_list_item_check::close,
@@ -563,6 +594,7 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
         Name::HtmlTextLineEndingAfterPrefix => construct::html_text::line_ending_after_prefix,
 
         Name::LabelStart => construct::partial_label::start,
+        Name::LabelAtMarker => construct::partial_label::at_marker,
         Name::LabelAtBreak => construct::partial_label::at_break,
         Name::LabelEolAfter => construct::partial_label::eol_after,
         Name::LabelAtBlankLine => construct::partial_label::at_blank_line,
@@ -591,6 +623,7 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
 
         Name::LabelStartImageStart => construct::label_start_image::start,
         Name::LabelStartImageOpen => construct::label_start_image::open,
+        Name::LabelStartImageAfter => construct::label_start_image::after,
         Name::LabelStartLinkStart => construct::label_start_link::start,
 
         Name::ListItemStart => construct::list_item::start,
@@ -633,6 +666,7 @@ pub fn call(tokenizer: &mut Tokenizer, name: Name) -> State {
         Name::TextBefore => construct::text::before,
         Name::TextBeforeHtml => construct::text::before_html,
         Name::TextBeforeHardBreakEscape => construct::text::before_hard_break_escape,
+        Name::TextBeforeLabelStartLink => construct::text::before_label_start_link,
         Name::TextBeforeData => construct::text::before_data,
 
         Name::ThematicBreakStart => construct::thematic_break::start,
diff --git a/src/tokenizer.rs b/src/tokenizer.rs
index 83514cb..c6a209b 100644
--- a/src/tokenizer.rs
+++ b/src/tokenizer.rs
@@ -28,6 +28,8 @@ pub enum Container {
     BlockQuote,
     /// [List item][crate::construct::list_item].
     ListItem,
+    /// [GFM: Footnote definition][crate::construct::gfm_footnote_definition].
+    GfmFootnoteDefinition,
 }
 
 /// Info used to tokenize a container.
@@ -56,9 +58,53 @@ enum ByteAction {
     Insert(u8),
 }
 
+/// Label start kind.
+#[derive(Debug, PartialEq, Eq)]
+pub enum LabelKind {
+    /// Label (image) start.
+    ///
+    /// ```markdown
+    /// > | a ![b] c
+    ///       ^^
+    /// ```
+    ///
+    /// Construct: [Label start (image)][crate::construct::label_start_image].
+    Image,
+    /// Label (image) link.
+    ///
+    /// ```markdown
+    /// > | a [b] c
+    ///       ^
+    /// ```
+    ///
+    /// Construct: [Label start (link)][crate::construct::label_start_link].
+    Link,
+    /// GFM: Label (footnote) link.
+    ///
+    /// ```markdown
+    /// > | a [^b] c
+    ///       ^^
+    /// ```
+    ///
+    /// Construct: [GFM: Label start (footnote)][crate::construct::gfm_label_start_footnote].
+    GfmFootnote,
+    /// GFM: Label (footnote) link, not matching a footnote definition, so
+    /// handled as a label (link) start.
+    ///
+    /// ```markdown
+    /// > | a [^b](c) d
+    ///       ^^
+    /// ```
+    ///
+    /// Construct: [Label end][crate::construct::label_end].
+    GfmUndefinedFootnote,
+}
+
 /// Label start, looking for an end.
 #[derive(Debug)]
 pub struct LabelStart {
+    /// Kind of start.
+    pub kind: LabelKind,
     /// Indices of where the label starts and ends in `events`.
     pub start: (usize, usize),
     /// A boolean used internally to figure out if a (link) label start can’t
@@ -71,6 +117,7 @@ pub struct LabelStart {
 /// Valid label.
 #[derive(Debug)]
 pub struct Label {
+    pub kind: LabelKind,
     /// Indices of label start.
     pub start: (usize, usize),
     /// Indices of label end.
@@ -174,8 +221,10 @@ pub struct TokenizeState<'a> {
     /// Used when tokenizing [text content][crate::construct::text].
     pub labels: Vec<Label>,
 
-    /// List of defined identifiers.
+    /// List of defined definition identifiers.
     pub definitions: Vec<String>,
+    /// List of defined GFM footnote definition identifiers.
+    pub gfm_footnote_definitions: Vec<String>,
 
     /// Whether to connect events.
     pub connect: bool,
@@ -288,6 +337,7 @@ impl<'a> Tokenizer<'a> {
                 document_child: None,
                 document_at_first_paragraph_of_list_item: false,
                 definitions: vec![],
+                gfm_footnote_definitions: vec![],
                 end: 0,
                 label_starts: vec![],
                 label_starts_loose: vec![],
diff --git a/tests/gfm_footnote.rs b/tests/gfm_footnote.rs
new file mode 100644
index 0000000..8c6cc54
--- /dev/null
+++ b/tests/gfm_footnote.rs
@@ -0,0 +1,1530 @@
+extern crate micromark;
+use micromark::{micromark, micromark_with_options, Constructs, Options};
+use pretty_assertions::assert_eq;
+
+#[test]
+fn gfm_footnote() {
+    let gfm = Options {
+        constructs: Constructs::gfm(),
+        ..Options::default()
+    };
+
+    assert_eq!(
+        micromark("A call.[^a]\n\n[^a]: whatevs"),
+        "<p>A call.<a href=\"whatevs\">^a</a></p>\n",
+        "should ignore footnotes by default"
+    );
+
+    assert_eq!(
+        micromark_with_options("A call.[^a]\n\n[^a]: whatevs", &gfm),
+        "<p>A call.<sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a\">
+<p>whatevs <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support footnotes"
+    );
+    assert_eq!(
+        micromark_with_options(
+            "Noot.[^a]\n\n[^a]: dingen",
+            &Options {
+                constructs: Constructs::gfm(),
+                gfm_footnote_label: Some("Voetnoten".to_string()),
+                gfm_footnote_back_label: Some("Terug naar de inhoud".to_string()),
+                ..Options::default()
+            }
+        ),
+        "<p>Noot.<sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Voetnoten</h2>
+<ol>
+<li id=\"user-content-fn-a\">
+<p>dingen <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Terug naar de inhoud\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support `options.gfm_footnote_label`, `options.gfm_footnote_back_label`"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            "[^a]\n\n[^a]: b",
+            &Options {
+                constructs: Constructs::gfm(),
+                gfm_footnote_label_tag_name: Some("h1".to_string()),
+                ..Options::default()
+            }
+        ),
+        "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h1 id=\"footnote-label\" class=\"sr-only\">Footnotes</h1>
+<ol>
+<li id=\"user-content-fn-a\">
+<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support `options.gfm_footnote_label_tag_name`"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            "[^a]\n\n[^a]: b",
+            &Options {
+                constructs: Constructs::gfm(),
+                gfm_footnote_label_attributes: Some("class=\"footnote-heading\"".to_string()),
+                ..Options::default()
+            }
+        ),
+        "<p><sup><a href=\"#user-content-fn-a\" id=\"user-content-fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"footnote-heading\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a\">
+<p>b <a href=\"#user-content-fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support `options.gfm_footnote_label_attributes`"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            "[^a]\n\n[^a]: b",
+            &Options {
+                constructs: Constructs::gfm(),
+                gfm_footnote_clobber_prefix: Some("".to_string()),
+                ..Options::default()
+            }
+        ),
+        "<p><sup><a href=\"#fn-a\" id=\"fnref-a\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"fn-a\">
+<p>b <a href=\"#fnref-a\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support `options.gfm_footnote_clobber_prefix`"
+    );
+
+    assert_eq!(
+        micromark_with_options("A paragraph.\n\n[^a]: whatevs", &gfm),
+        "<p>A paragraph.</p>\n",
+        "should ignore definitions w/o calls"
+    );
+
+    assert_eq!(
+        micromark_with_options("a[^b]", &gfm),
+        "<p>a[^b]</p>",
+        "should ignore calls w/o definitions"
+    );
+
+    assert_eq!(
+        micromark_with_options("a[^b]\n\n[^b]: c\n[^b]: d", &gfm),
+        "<p>a<sup><a href=\"#user-content-fn-b\" id=\"user-content-fnref-b\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-b\">
+<p>c <a href=\"#user-content-fnref-b\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should use the first of duplicate definitions"
+    );
+
+    assert_eq!(
+        micromark_with_options("a[^b], c[^b]\n\n[^b]: d", &gfm),
+        "<p>a<sup><a href=\"#user-content-fn-b\" id=\"user-content-fnref-b\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>, c<sup><a href=\"#user-content-fn-b\" id=\"user-content-fnref-b-2\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup></p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-b\">
+<p>d <a href=\"#user-content-fnref-b\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a> <a href=\"#user-content-fnref-b-2\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩<sup>2</sup></a></p>
+</li>
+</ol>
+</section>
+",
+        "should supports multiple calls to the same definition"
+    );
+
+    assert_eq!(
+        micromark_with_options("![^a](b)", &gfm),
+        "<p>!<a href=\"b\">^a</a></p>",
+        "should not support images starting w/ `^` (but see it as a link?!, 1)"
+    );
+
+    assert_eq!(
+        micromark_with_options("![^a][b]\n\n[b]: c", &gfm),
+        "<p>!<a href=\"c\">^a</a></p>\n",
+        "should not support images starting w/ `^` (but see it as a link?!, 2)"
+    );
+
+    assert_eq!(
+        micromark_with_options("[^]()", &gfm),
+        "<p><a href=\"\">^</a></p>",
+        "should support an empty link with caret"
+    );
+
+    assert_eq!(
+        micromark_with_options("![^]()", &gfm),
+        "<p>!<a href=\"\">^</a></p>",
+        "should support an empty image with caret (as link)"
+    );
+
+    // <https://github.com/github/cmark-gfm/issues/239>
+    assert_eq!(
+        micromark_with_options("Call.[^a\\+b].\n\n[^a\\+b]: y", &gfm),
+        "<p>Call.<sup><a href=\"#user-content-fn-a%5C+b\" id=\"user-content-fnref-a%5C+b\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a%5C+b\">
+<p>y <a href=\"#user-content-fnref-a%5C+b\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support a character escape in a call / definition"
+    );
+
+    assert_eq!(
+        micromark_with_options("Call.[^a&copy;b].\n\n[^a&copy;b]: y", &gfm),
+        "<p>Call.<sup><a href=\"#user-content-fn-a&amp;copy;b\" id=\"user-content-fnref-a&amp;copy;b\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a&amp;copy;b\">
+<p>y <a href=\"#user-content-fnref-a&amp;copy;b\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support a character reference in a call / definition"
+    );
+
+    // <https://github.com/github/cmark-gfm/issues/239>
+    // <https://github.com/github/cmark-gfm/issues/240>
+    assert_eq!(
+        micromark_with_options("Call.[^a\\]b].\n\n[^a\\]b]: y", &gfm),
+        "<p>Call.<sup><a href=\"#user-content-fn-a%5C%5Db\" id=\"user-content-fnref-a%5C%5Db\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a%5C%5Db\">
+<p>y <a href=\"#user-content-fnref-a%5C%5Db\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support a useful character escape in a call / definition"
+    );
+
+    assert_eq!(
+        micromark_with_options("Call.[^a&#91;b].\n\n[^a&#91;b]: y", &gfm),
+        "<p>Call.<sup><a href=\"#user-content-fn-a&amp;#91;b\" id=\"user-content-fnref-a&amp;#91;b\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-a&amp;#91;b\">
+<p>y <a href=\"#user-content-fnref-a&amp;#91;b\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support a useful character reference in a call / definition"
+    );
+
+    assert_eq!(
+        micromark_with_options("Call.[^a\\+b].\n\n[^a+b]: y", &gfm),
+        "<p>Call.[^a+b].</p>\n",
+        "should match calls to definitions on the source of the label, not on resolved escapes"
+    );
+
+    assert_eq!(
+        micromark_with_options("Call.[^a&#91;b].\n\n[^a\\[b]: y", &gfm),
+        "<p>Call.[^a[b].</p>\n",
+        "should match calls to definitions on the source of the label, not on resolved references"
+    );
+
+    assert_eq!(
+        micromark_with_options("[^1].\n\n[^1]: a\nb", &gfm),
+        "<p><sup><a href=\"#user-content-fn-1\" id=\"user-content-fnref-1\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-1\">
+<p>a
+b <a href=\"#user-content-fnref-1\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support lazyness (1)"
+    );
+
+    assert_eq!(
+        micromark_with_options("[^1].\n\n> [^1]: a\nb", &gfm),
+        "<p><sup><a href=\"#user-content-fn-1\" id=\"user-content-fnref-1\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<blockquote>
+</blockquote>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-1\">
+<p>a
+b <a href=\"#user-content-fnref-1\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support lazyness (2)"
+    );
+
+    assert_eq!(
+        micromark_with_options("[^1].\n\n> [^1]: a\n> b", &gfm),
+        "<p><sup><a href=\"#user-content-fn-1\" id=\"user-content-fnref-1\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<blockquote>
+</blockquote>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-1\">
+<p>a
+b <a href=\"#user-content-fnref-1\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+",
+        "should support lazyness (3)"
+    );
+
+    assert_eq!(
+        micromark_with_options("[^1].\n\n[^1]: a\n\n    > b", &gfm),
+        "<p><sup><a href=\"#user-content-fn-1\" id=\"user-content-fnref-1\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-1\">
+<p>a</p>
+<blockquote>
+<p>b</p>
+</blockquote>
+<a href=\"#user-content-fnref-1\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a>
+</li>
+</ol>
+</section>
+",
+        "should support lazyness (4)"
+    );
+
+    // 999 `x` characters.
+    let max = "x".repeat(999);
+
+    assert_eq!(
+        micromark_with_options(format!("Call.[^{}].\n\n[^{}]: y", max, max).as_str(), &gfm),
+        format!("<p>Call.<sup><a href=\"#user-content-fn-{}\" id=\"user-content-fnref-{}\" data-footnote-ref=\"\" aria-describedby=\"footnote-label\">1</a></sup>.</p>
+<section data-footnotes=\"\" class=\"footnotes\"><h2 id=\"footnote-label\" class=\"sr-only\">Footnotes</h2>
+<ol>
+<li id=\"user-content-fn-{}\">
+<p>y <a href=\"#user-content-fnref-{}\" data-footnote-backref=\"\" class=\"data-footnote-backref\" aria-label=\"Back to content\">↩</a></p>
+</li>
+</ol>
+</section>
+", max, max, max, max),
+        "should support 999 characters in a call / definition"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            format!("Call.[^a{}].\n\n[^a{}]: y", max, max).as_str(),
+            &gfm
+        ),
+        format!("<p>Call.[^a{}].</p>\n<p>[^a{}]: y</p>", max, max),
+        "should not support 1000 characters in a call / definition"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"a![i](#)
+a\![i](#)
+a![i][]
+a![^1]
+[^1]
+^1]
+
+[^1]: b
+
+[i]: c"###,
+            &gfm
+        ),
+        r###"<p>a<img src="#" alt="i" />
+a!<a href="#">i</a>
+a<img src="c" alt="i" />
+a!<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup>
+<sup><a href="#user-content-fn-1" id="user-content-fnref-1-2" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup>
+^1]</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>b <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a> <a href="#user-content-fnref-1-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩<sup>2</sup></a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match bang/caret interplay like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options("a![^1]", &gfm),
+        "<p>a![^1]</p>",
+        "should match bang/caret interplay (undefined) like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"a![^1]
+
+[^1]: b
+"###,
+            &gfm
+        ),
+        r###"<p>a!<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>b <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match bang/caret like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Calls may not be empty: [^].
+
+Calls cannot contain whitespace only: [^ ].
+
+Calls cannot contain whitespace at all: [^ ], [^	], [^
+].
+
+Calls can contain other characters, such as numbers [^1234567890], or [^^]
+even another caret.
+
+[^]: empty
+
+[^ ]: space
+
+[^	]: tab
+
+[^
+]&#x3A; line feed
+
+[^1234567890]: numbers
+
+[^^]: caret
+"###,
+            &gfm
+        ),
+        r###"<p>Calls may not be empty: <a href="empty">^</a>.</p>
+<p>Calls cannot contain whitespace only: <a href="empty">^ </a>.</p>
+<p>Calls cannot contain whitespace at all: <a href="empty">^ </a>, <a href="empty">^	</a>, <a href="empty">^
+</a>.</p>
+<p>Calls can contain other characters, such as numbers <sup><a href="#user-content-fn-1234567890" id="user-content-fnref-1234567890" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup>, or <sup><a href="#user-content-fn-%5E" id="user-content-fnref-%5E" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup>
+even another caret.</p>
+<p><a href="empty">^
+</a>: line feed</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1234567890">
+<p>numbers <a href="#user-content-fnref-1234567890" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-%5E">
+<p>caret <a href="#user-content-fnref-%5E" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match calls like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^*emphasis*]
+
+[^**strong**]
+
+[^`code`]
+
+[^www.example.com]
+
+[^https://example.com]
+
+[^://example.com]
+
+[^[link](#)]
+
+[^![image](#)]
+
+[^*emphasis*]: a
+
+[^**strong**]: a
+
+[^`code`]: a
+
+[^www.example.com]: a
+
+[^https://example.com]: a
+
+[^://example.com]: a
+
+[^[link](#)]: a
+
+[^![image](#)]: a
+"###,
+            &gfm
+        ),
+        // Note:
+        // * GH does not support colons.
+        //   See: <https://github.com/github/cmark-gfm/issues/250>
+        //   Here identifiers that include colons *do* work (so they’re added below).
+        // * GH does not support footnote-like brackets around an image.
+        //   See: <https://github.com/github/cmark-gfm/issues/275>
+        //   Here images are fine.
+        r###"<p><sup><a href="#user-content-fn-*emphasis*" id="user-content-fnref-*emphasis*" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>
+<p><sup><a href="#user-content-fn-**strong**" id="user-content-fnref-**strong**" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup></p>
+<p><sup><a href="#user-content-fn-%60code%60" id="user-content-fnref-%60code%60" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup></p>
+<p><sup><a href="#user-content-fn-www.example.com" id="user-content-fnref-www.example.com" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<p><sup><a href="#user-content-fn-https://example.com" id="user-content-fnref-https://example.com" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup></p>
+<p><sup><a href="#user-content-fn-://example.com" id="user-content-fnref-://example.com" data-footnote-ref="" aria-describedby="footnote-label">6</a></sup></p>
+<p>[^<a href="#">link</a>]</p>
+<p>[^<img src="#" alt="image" />]</p>
+<p>[^<a href="#">link</a>]: a</p>
+<p>[^<img src="#" alt="image" />]: a</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-*emphasis*">
+<p>a <a href="#user-content-fnref-*emphasis*" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-**strong**">
+<p>a <a href="#user-content-fnref-**strong**" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-%60code%60">
+<p>a <a href="#user-content-fnref-%60code%60" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-www.example.com">
+<p>a <a href="#user-content-fnref-www.example.com" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-https://example.com">
+<p>a <a href="#user-content-fnref-https://example.com" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-://example.com">
+<p>a <a href="#user-content-fnref-://example.com" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match construct identifiers like GitHub (except for its bugs)"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1][^2][^3][^4]
+
+> [^1]: Defined in a block quote.
+>
+> More.
+[^2]: Directly after a block quote.
+
+* [^3]: Defined in a list item.
+
+  More.
+[^4]: Directly after a list item.
+"###,
+            &gfm
+        ),
+        r###"<p>Call<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<blockquote>
+<p>More.</p>
+</blockquote>
+<ul>
+<li>
+<p>More.</p>
+</li>
+</ul>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Defined in a block quote. <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Directly after a block quote. <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>Defined in a list item. <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>Directly after a list item. <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match containers like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^1][^2][^3][^4]
+
+[^1]: Paragraph
+…continuation
+
+# Heading
+
+[^2]: Paragraph
+…continuation
+
+    “code”, which is paragraphs…
+
+    …because of the indent!
+
+[^3]: Paragraph
+…continuation
+
+> block quote
+
+[^4]: Paragraph
+…continuation
+
+- list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<h1>Heading</h1>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<ul>
+<li>list</li>
+</ul>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph
+…continuation</p>
+<p>“code”, which is paragraphs…</p>
+<p>…because of the indent! <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match continuation like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1][^2][^3][^4][^5].
+
+[^1]:
+    ---
+
+[^2]:
+    Paragraph.
+
+[^3]:
+Lazy?
+
+[^4]:
+
+    Another blank.
+
+[^5]:
+
+Lazy!
+"###,
+            &gfm
+        ),
+        r###"<p>Call<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup><sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup>.</p>
+<p>Lazy?</p>
+<p>Lazy!</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<hr />
+<a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph. <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-4">
+<p>Another blank. <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+</ol>
+</section>
+"###,
+        "should match definitions initial blank like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Note![^0][^1][^2][^3][^4][^5][^6][^7][^8][^9][^10]
+
+[^0]: alpha
+
+[^1]: bravo
+
+[^2]: charlie
+    indented delta
+
+[^3]:    echo
+
+[^4]:     foxtrot
+
+[^5]:> golf
+
+[^6]:    > hotel
+
+[^7]:     > india
+
+[^8]: # juliett
+
+[^9]: ---
+
+[^10]:- - - kilo
+"###,
+            &gfm
+        ),
+        r###"<p>Note!<sup><a href="#user-content-fn-0" id="user-content-fnref-0" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup><sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">6</a></sup><sup><a href="#user-content-fn-6" id="user-content-fnref-6" data-footnote-ref="" aria-describedby="footnote-label">7</a></sup><sup><a href="#user-content-fn-7" id="user-content-fnref-7" data-footnote-ref="" aria-describedby="footnote-label">8</a></sup><sup><a href="#user-content-fn-8" id="user-content-fnref-8" data-footnote-ref="" aria-describedby="footnote-label">9</a></sup><sup><a href="#user-content-fn-9" id="user-content-fnref-9" data-footnote-ref="" aria-describedby="footnote-label">10</a></sup><sup><a href="#user-content-fn-10" id="user-content-fnref-10" data-footnote-ref="" aria-describedby="footnote-label">11</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-0">
+<p>alpha <a href="#user-content-fnref-0" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-1">
+<p>bravo <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>charlie
+indented delta <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>echo <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>foxtrot <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<blockquote>
+<p>golf</p>
+</blockquote>
+<a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-6">
+<blockquote>
+<p>hotel</p>
+</blockquote>
+<a href="#user-content-fnref-6" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-7">
+<blockquote>
+<p>india</p>
+</blockquote>
+<a href="#user-content-fnref-7" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-8">
+<h1>juliett</h1>
+<a href="#user-content-fnref-8" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-9">
+<hr />
+<a href="#user-content-fnref-9" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-10">
+<ul>
+<li>
+<ul>
+<li>
+<ul>
+<li>kilo</li>
+</ul>
+</li>
+</ul>
+</li>
+</ul>
+<a href="#user-content-fnref-10" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+</ol>
+</section>
+"###,
+        "should match definitions like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1][^1]
+
+[^1]: Recursion[^1][^1]
+"###,
+            &gfm
+        ),
+        r###"<p>Call<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-1" id="user-content-fnref-1-2" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Recursion<sup><a href="#user-content-fn-1" id="user-content-fnref-1-3" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-1" id="user-content-fnref-1-4" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup> <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a> <a href="#user-content-fnref-1-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩<sup>2</sup></a> <a href="#user-content-fnref-1-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩<sup>3</sup></a> <a href="#user-content-fnref-1-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩<sup>4</sup></a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match duplicate calls and recursion like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1]
+
+[^1]: a
+
+[^1]: b
+"###,
+            &gfm
+        ),
+        r###"<p>Call<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>a <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match duplicate definitions like GitHub"
+    );
+
+    // Note:
+    // * GH “supports” footnotes inside links.
+    //   This breaks an HTML parser, as it is not allowed.
+    //   See: <https://github.com/github/cmark-gfm/issues/275>
+    //   CommonMark has mechanisms in place to prevent links in links.
+    //   These mechanisms are in place here too.
+    assert_eq!(
+        micromark_with_options(
+            r###"*emphasis[^1]*
+
+**strong[^2]**
+
+`code[^3]`
+
+![image[^4]](#)
+
+[link[^5]](#)
+
+[^1]: a
+
+[^2]: b
+
+[^3]: c
+
+[^4]: d
+
+[^5]: e
+"###,
+            &gfm
+        ),
+        r###"<p><em>emphasis<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup></em></p>
+<p><strong>strong<sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup></strong></p>
+<p><code>code[^3]</code></p>
+<p><img src="#" alt="image" /></p>
+<p>[link<sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup>](#)</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>a <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>b <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>d <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<p>e <a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match footnotes in constructs like GitHub (without the bugs)"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"What are these![^1], ![^2][], and ![this][^3].
+
+[^1]: a
+
+[^2]: b
+
+[^3]: c
+"###,
+            &gfm
+        ),
+        r###"<p>What are these!<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup>, !<sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup>[], and ![this]<sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup>.</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>a <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>b <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>c <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match images/footnotes like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^0][^1][^2][^3][^4][^5]
+
+[^0]: Paragraph
+…continuation
+[^1]: Another
+[^2]: Paragraph
+…continuation
+# Heading
+[^3]: Paragraph
+…continuation
+    “code”, which is paragraphs…
+
+    …because of the indent!
+[^4]: Paragraph
+…continuation
+> block quote
+[^5]: Paragraph
+…continuation
+*   list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-0" id="user-content-fnref-0" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup><sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">6</a></sup></p>
+<h1>Heading</h1>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<ul>
+<li>list</li>
+</ul>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-0">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-0" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-1">
+<p>Another <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph
+…continuation
+“code”, which is paragraphs…</p>
+<p>…because of the indent! <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<p>Paragraph
+…continuation <a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match interrupt like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"What are these[^1], [^2][], and [this][^3].
+
+[^1]: a
+
+[^2]: b
+
+[^3]: c
+"###,
+            &gfm
+        ),
+        r###"<p>What are these<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup>, <sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup>[], and [this]<sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup>.</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>a <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>b <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>c <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match links/footnotes like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^1][^2][^3][^4]
+
+[^1]: Paragraph
+
+
+# Heading
+
+
+[^2]: Paragraph
+
+
+    “code”, which is paragraphs…
+
+
+    …because of the indent!
+
+
+[^3]: Paragraph
+
+
+> block quote
+
+
+[^4]: Paragraph
+
+
+- list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<h1>Heading</h1>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<ul>
+<li>list</li>
+</ul>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph</p>
+<p>“code”, which is paragraphs…</p>
+<p>…because of the indent! <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match many blank lines/no indent like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^1][^2][^3][^4]
+
+[^1]: Paragraph
+
+
+    # Heading
+
+
+[^2]: Paragraph
+
+
+        code
+
+
+        more code
+
+
+[^3]: Paragraph
+
+
+    > block quote
+
+
+[^4]: Paragraph
+
+
+    - list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph</p>
+<h1>Heading</h1>
+<a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph</p>
+<pre><code>code
+
+
+more code
+</code></pre>
+<a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph</p>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph</p>
+<ul>
+<li>list</li>
+</ul>
+<a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+</ol>
+</section>
+"###,
+        "should match many blank lines like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Note![^1][^2][^3][^4]
+
+- [^1]: Paragraph
+
+> [^2]: Paragraph
+
+[^3]: [^4]: Paragraph
+"###,
+            &gfm
+        ),
+        r###"<p>Note!<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<ul>
+<li></li>
+</ul>
+<blockquote>
+</blockquote>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match nest like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^1][^2][^3][^4]
+
+[^1]: Paragraph
+
+# Heading
+
+[^2]: Paragraph
+
+    “code”, which is paragraphs…
+
+    …because of the indent!
+
+[^3]: Paragraph
+
+> block quote
+
+[^4]: Paragraph
+
+- list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<h1>Heading</h1>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<ul>
+<li>list</li>
+</ul>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph</p>
+<p>“code”, which is paragraphs…</p>
+<p>…because of the indent! <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match normal blank lines/no indent like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"[^1][^2][^3][^4]
+
+[^1]: Paragraph
+
+    # Heading
+
+[^2]: Paragraph
+
+        code
+
+        more code
+
+[^3]: Paragraph
+
+    > block quote
+
+[^4]: Paragraph
+
+    - list
+"###,
+            &gfm
+        ),
+        r###"<p><sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup></p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Paragraph</p>
+<h1>Heading</h1>
+<a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-2">
+<p>Paragraph</p>
+<pre><code>code
+
+more code
+</code></pre>
+<a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-3">
+<p>Paragraph</p>
+<blockquote>
+<p>block quote</p>
+</blockquote>
+<a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-4">
+<p>Paragraph</p>
+<ul>
+<li>list</li>
+</ul>
+<a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+</ol>
+</section>
+"###,
+        "should match normal blank lines like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Here is a footnote reference,[^1] and another.[^longnote]
+
+[^1]: Here is the footnote.
+
+[^longnote]: Here’s one with multiple blocks.
+
+    Subsequent paragraphs are indented to show that they
+belong to the previous footnote.
+
+        { some.code }
+
+    The whole paragraph can be indented, or just the first
+    line.  In this way, multi-paragraph footnotes work like
+    multi-paragraph list items.
+
+This paragraph won’t be part of the note, because it
+isn’t indented.
+"###,
+            &gfm
+        ),
+        r###"<p>Here is a footnote reference,<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup> and another.<sup><a href="#user-content-fn-longnote" id="user-content-fnref-longnote" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup></p>
+<p>This paragraph won’t be part of the note, because it
+isn’t indented.</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>Here is the footnote. <a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-longnote">
+<p>Here’s one with multiple blocks.</p>
+<p>Subsequent paragraphs are indented to show that they
+belong to the previous footnote.</p>
+<pre><code>{ some.code }
+</code></pre>
+<p>The whole paragraph can be indented, or just the first
+line.  In this way, multi-paragraph footnotes work like
+multi-paragraph list items. <a href="#user-content-fnref-longnote" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match pandoc like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1][^2][^3][^4][^5][^6][^7][^8][^9][^10][^11][^12].
+
+     [^1]: 5
+
+    [^2]: 4
+
+   [^3]: 3
+
+  [^4]: 2
+
+ [^5]: 1
+
+[^6]: 0
+
+***
+
+   [^7]: 3
+
+     5
+
+   [^8]: 3
+
+    4
+
+   [^9]: 3
+
+   3
+
+  [^10]: 2
+
+     5
+
+  [^11]: 2
+
+    4
+
+  [^12]: 2
+
+   3
+"###,
+            &gfm
+        ),
+        r###"<p>Call[^1][^2]<sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-6" id="user-content-fnref-6" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup><sup><a href="#user-content-fn-7" id="user-content-fnref-7" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup><sup><a href="#user-content-fn-8" id="user-content-fnref-8" data-footnote-ref="" aria-describedby="footnote-label">6</a></sup><sup><a href="#user-content-fn-9" id="user-content-fnref-9" data-footnote-ref="" aria-describedby="footnote-label">7</a></sup><sup><a href="#user-content-fn-10" id="user-content-fnref-10" data-footnote-ref="" aria-describedby="footnote-label">8</a></sup><sup><a href="#user-content-fn-11" id="user-content-fnref-11" data-footnote-ref="" aria-describedby="footnote-label">9</a></sup><sup><a href="#user-content-fn-12" id="user-content-fnref-12" data-footnote-ref="" aria-describedby="footnote-label">10</a></sup>.</p>
+<pre><code> [^1]: 5
+
+[^2]: 4
+</code></pre>
+<hr />
+<p>3</p>
+<p>3</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-3">
+<p>3 <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>2 <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<p>1 <a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-6">
+<p>0 <a href="#user-content-fnref-6" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-7">
+<p>3</p>
+<p>5 <a href="#user-content-fnref-7" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-8">
+<p>3</p>
+<p>4 <a href="#user-content-fnref-8" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-9">
+<p>3 <a href="#user-content-fnref-9" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-10">
+<p>2</p>
+<p>5 <a href="#user-content-fnref-10" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-11">
+<p>2</p>
+<p>4 <a href="#user-content-fnref-11" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-12">
+<p>2 <a href="#user-content-fnref-12" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match prefix before like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Call[^1][^2][^3][^4][^5][^6][^7][^8][^9].
+
+[^1]: a
+
+        8
+
+[^2]: a
+
+       7
+
+[^3]: a
+
+      6
+
+[^4]: a
+
+     5
+
+[^5]: a
+
+    4
+
+[^6]: a
+
+   3
+
+[^7]: a
+
+  2
+
+[^8]: a
+
+ 1
+
+[^9]: a
+
+0
+"###,
+            &gfm
+        ),
+        r###"<p>Call<sup><a href="#user-content-fn-1" id="user-content-fnref-1" data-footnote-ref="" aria-describedby="footnote-label">1</a></sup><sup><a href="#user-content-fn-2" id="user-content-fnref-2" data-footnote-ref="" aria-describedby="footnote-label">2</a></sup><sup><a href="#user-content-fn-3" id="user-content-fnref-3" data-footnote-ref="" aria-describedby="footnote-label">3</a></sup><sup><a href="#user-content-fn-4" id="user-content-fnref-4" data-footnote-ref="" aria-describedby="footnote-label">4</a></sup><sup><a href="#user-content-fn-5" id="user-content-fnref-5" data-footnote-ref="" aria-describedby="footnote-label">5</a></sup><sup><a href="#user-content-fn-6" id="user-content-fnref-6" data-footnote-ref="" aria-describedby="footnote-label">6</a></sup><sup><a href="#user-content-fn-7" id="user-content-fnref-7" data-footnote-ref="" aria-describedby="footnote-label">7</a></sup><sup><a href="#user-content-fn-8" id="user-content-fnref-8" data-footnote-ref="" aria-describedby="footnote-label">8</a></sup><sup><a href="#user-content-fn-9" id="user-content-fnref-9" data-footnote-ref="" aria-describedby="footnote-label">9</a></sup>.</p>
+<p>3</p>
+<p>2</p>
+<p>1</p>
+<p>0</p>
+<section data-footnotes="" class="footnotes"><h2 id="footnote-label" class="sr-only">Footnotes</h2>
+<ol>
+<li id="user-content-fn-1">
+<p>a</p>
+<pre><code>8
+</code></pre>
+<a href="#user-content-fnref-1" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a>
+</li>
+<li id="user-content-fn-2">
+<p>a</p>
+<p>7 <a href="#user-content-fnref-2" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-3">
+<p>a</p>
+<p>6 <a href="#user-content-fnref-3" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-4">
+<p>a</p>
+<p>5 <a href="#user-content-fnref-4" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-5">
+<p>a</p>
+<p>4 <a href="#user-content-fnref-5" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-6">
+<p>a <a href="#user-content-fnref-6" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-7">
+<p>a <a href="#user-content-fnref-7" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-8">
+<p>a <a href="#user-content-fnref-8" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+<li id="user-content-fn-9">
+<p>a <a href="#user-content-fnref-9" data-footnote-backref="" class="data-footnote-backref" aria-label="Back to content">↩</a></p>
+</li>
+</ol>
+</section>
+"###,
+        "should match prefix like GitHub"
+    );
+
+    assert_eq!(
+        micromark_with_options(
+            r###"Here is a short reference,[1], a collapsed one,[2][], and a full [one][3].
+
+[1]: a
+
+[2]: b
+
+[3]: c
+"###,
+            &gfm
+        ),
+        r###"<p>Here is a short reference,<a href="a">1</a>, a collapsed one,<a href="b">2</a>, and a full <a href="c">one</a>.</p>
+"###,
+        "should match references and definitions like GitHub"
+    );
+}