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diff --git a/src/construct/blank_line.rs b/src/construct/blank_line.rs
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+//! Blank lines are a construct that occurs in the flow content type.
+//!
+//! They’re formed with the following BNF:
+//!
+//! ```bnf
+//! blank_line ::= *(' ' '\t')
+//! ```
+//!
+//! Blank lines are sometimes needed, such as to differentiate a paragraph
+//! from another paragraph.
+//! In several cases, blank lines are not needed between flow constructs,
+//! such as between two headings.
+//! Sometimes, whether blank lines are present, changes the behavior of how
+//! HTML is rendered, such as whether blank lines are present between list
+//! items in a list.
+//! More than one blank line is never needed in `CommonMark`.
+//!
+//! Because blank lines can be empty (line endings are not considered part of
+//! it), and events cannot be empty, blank lines are not present as a token.
+//!
+//! ## References
+//!
+//! *   [`blank-line.js` in `micromark`](https://github.com/micromark/micromark/blob/main/packages/micromark-core-commonmark/dev/lib/blank-line.js)
+//! *   [*§ 4.9 Blank lines* in `CommonMark`](https://spec.commonmark.org/0.30/#blank-lines)
+//!
+//! <!-- To do: link `flow`, `heading`, `list`, `paragraph` -->
+
+use crate::construct::partial_whitespace::start as whitespace;
+use crate::tokenizer::{Code, State, StateFnResult, TokenType, Tokenizer};
+
+/// Start of a blank line.
+///
+/// Note: `␠` represents a space character.
+///
+/// ```markdown
+/// |␠␠
+/// |
+/// ```
+pub fn start(tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    tokenizer.attempt(
+        |tokenizer, code| whitespace(tokenizer, code, TokenType::BlankLineWhitespace),
+        |_ok| Box::new(after),
+    )(tokenizer, code)
+}
+
+/// After zero or more spaces or tabs, before a line ending or EOF.
+///
+/// Note: `␠` represents a space character.
+///
+/// ```markdown
+/// |␠␠
+/// |
+/// ```
+fn after(_tokenizer: &mut Tokenizer, code: Code) -> StateFnResult {
+    match code {
+        Code::None | Code::CarriageReturnLineFeed | Code::Char('\n' | '\r') => {
+            (State::Ok, Some(vec![code]))
+        }
+        _ => (State::Nok, None),
+    }
+}