Markdown-Gfm

# GitHub Flavored Markdown (GFM)

Apply these rules per the [GFM spec](https://github.github.com/gfm/) when writing or reviewing `.md` files. GFM is a strict superset of CommonMark. GFM spec for reference only. Do not download GFM Spec.

Preliminaries

- A line ends at a newline (`U+000A`), carriage return (`U+000D`), or end of file. A blank line contains only spaces or tabs.

- Tabs behave as 4-space tab stops for block structure but are not expanded in content.

- Replace `U+0000` with the replacement character `U+FFFD`.

Leaf Blocks

- **Thematic breaks**: 3+ matching `-`, `_`, or `*` characters on a line with 0–3 spaces indent. No other characters on the line. Can interrupt a paragraph.

- **ATX headings**: 1–6 `#` characters followed by a space or end of line. Optional closing `#` sequence (preceded by a space). 0–3 spaces indent allowed.

- **Setext headings**: Text underlined with `=` (level 1) or `-` (level 2). Cannot interrupt a paragraph — blank line required after a preceding paragraph.

- **Indented code blocks**: Lines indented 4+ spaces. Cannot interrupt a paragraph. Content is literal text, not parsed as Markdown.

- **Fenced code blocks**: Open with 3+ backticks or tildes (do not mix). Closing fence must use same character with at least the same count. Specify language identifier after the opening fence. Content is literal text.

- **HTML blocks**: Seven types defined by start/end tag conditions. Types 1–6 can interrupt paragraphs; type 7 cannot. Content is passed through as raw HTML.

- Type 1: `<script>`, `<pre>`, or `<style>` (case-insensitive) — ends at matching closing tag.

- Type 2: `<!--` comment — ends at `-->`.

- Type 3: `<?` processing instruction — ends at `?>`.

- Type 4: `<!` + uppercase letter (e.g., `<!DOCTYPE>`) — ends at `>`.

- Type 5: `<![CDATA[` — ends at `]]>`.

- Type 6: Block-level HTML tags (`<div>`, `<table>`, `<p>`, `<h1>`–`<h6>`, `<ul>`, `<ol>`, `<section>`, etc.) — ends at a blank line.

- Type 7: Any other complete open or closing tag on its own line — ends at a blank line. Cannot interrupt a paragraph.

- **Link reference definitions**: `[label]: destination "title"`. Case-insensitive label matching. First definition wins for duplicate labels. Cannot interrupt a paragraph.

- **Paragraphs**: Consecutive non-blank lines not interpretable as other block constructs. Leading spaces up to 3 are stripped.

- **Blank lines**: Ignored between blocks; determine whether a list is tight or loose.

- **Tables** *(extension)*: Header row, delimiter row (`---`, `:---:`, `---:`), zero or more data rows. Delimit cells with `|`. Escape literal pipe as `\|`. Header and delimiter must have matching column count. Broken at first blank line or other block-level structure.

Container Blocks

- **Block quotes**: Lines prefixed with `>` (optionally followed by a space). Lazy continuation allowed for paragraph text only. A blank line separates consecutive block quotes.

- **List items**: Bullet markers (`-`, `+`, `*`) or ordered markers (1–9 digits + `.` or `)`). Content column determined by marker width + spaces to first non-whitespace. Sublists must be indented to the content column. An ordered list interrupting a paragraph must start with `1`.

- **Task list items** *(extension)*: `- [ ]` (unchecked) or `- [x]` (checked) at the start of a list item paragraph. Space between `-` and `[` is required. May be nested.

- **Lists**: Sequence of same-type list items. Changing bullet character or ordered delimiter starts a new list. A list is loose if any item is separated by a blank line.

Inlines

- **Backslash escapes**: `\` before any ASCII punctuation character renders the literal character. Not recognized in code spans, code blocks, or autolinks.

- **Entity and numeric character references**: `&amp;`, `&#123;`, `&#x7B;` — valid HTML5 entities. Not recognized in code spans or code blocks. Cannot replace structural characters.

- **Code spans**: Backtick-delimited inline code. Line endings convert to s