Feature Comparison

CommonMark defines everyday Markdown: headings, lists, links, emphasis, and code. GFM (GitHub Flavored Markdown) adds features such as tables and checklists. The groups below spell out those names and show useful extras. Footnotes and alert boxes are listed separately from the published GFM extensions.

✓ = built in, possibly requiring an option or Cargo feature. — = no built-in support. Qualifiers describe narrower support. This is a capability inventory, not a claim of identical output or complete specification conformance. It covers the native cores from the benchmark, with v2 updated to the current local core; these are not the options enabled during timing. Exact versions, source references, and scope.

Feature / what it does	Ferromark v1	Ferromark v2	OX-Content	pulldown-cmark	md4c	Bun native
CommonMark — everyday Markdown
Headings — # Title through ###### Title	✓	✓	✓	✓	✓	✓
Paragraphs and line breaks	✓	✓	✓	✓	✓	✓
Bold and italic — **bold**, *italic*	✓	✓	✓	✓	✓	✓
Numbered, bulleted, and nested lists	✓	✓	✓	✓	✓	✓
Block quotes — > quoted text	✓	✓	✓	✓	✓	✓
Horizontal separators — ---	✓	✓	✓	✓	✓	✓
Links, images, and reusable reference links	✓	✓	✓	✓	✓	✓
Inline code, fenced code blocks, and indented code	✓	✓	✓	✓	✓	✓
Raw HTML inside Markdown — <details>…</details>	✓	✓	✓	✓	✓	✓
GFM — GitHub-style extensions
Tables with left/center/right column alignment	✓	✓	✓	✓	✓	✓
Checklists — - [x] done, - [ ] pending	✓	✓	✓	✓	✓	✓
Crossed-out text — ~~removed~~	✓	✓	✓	✓	✓	✓
Single-tilde crossed-out text — ~removed~	—	✓	—	✓	✓	✓
Turn bare URLs/emails into links — www.example.com	✓	✓	✓	—	✓	✓
GFM HTML tag filter — filter its specified tag list	✓	✓	✓	—	—	✓
Writing extras — beyond CommonMark/GFM
Reference footnotes — [^note] plus a definition	✓	✓	✓	✓	✓	—
Inline footnotes — ^[note written here]	✓	✓	—	—	—	—
Definition lists — a term followed by : explanation	✓	✓	✓	✓	—	—
Math notation — $x^2$, $$…$$	Syntax	Syntax	Syntax	Syntax	Syntax	Syntax
Superscript — x^2^	✓	✓	✓	✓	✓	—
Subscript — H~2~O	✓	✓	✓	✓	✓	—
Highlighted text — ==important==	✓	✓	—	—	✓	—
Wiki links — [[Page]]	—	✓	✓	✓	Custom tag	Custom tag
Smart punctuation — curly quotes and ellipses	—	—	✓	✓	—	—
Extract frontmatter — metadata between --- or +++	✓	✓	—	✓	—	—
Source-only line comments — hide // note lines from HTML	✓	✓	—	—	—	—
Table layout — beyond GFM
Merged table cells — one cell spans several columns	✓	✓	—	—	—	—
Numeric column-width hints — proportions from delimiter dashes	✓	—	—	—	—	—
Table IDs/classes — style a whole table with CSS	—	✓	—	—	—	—
Table captions — a label with inline Markdown formatting	—	With ID/class	—	—	—	—
Column classes for CSS widths — col-1, col-2, etc.	—	✓	—	—	—	—
Column names from headers — “Netto Preis” → col-name-netto-preis	—	✓	—	—	—	—
Documentation and navigation
Automatic heading IDs — link directly to a section	✓	✓	✓	—	—	✓
Explicit heading IDs/classes — # Title {#id .class}	—	✓	✓	✓	—	—
Alert boxes — > [!NOTE], > [!WARNING]	✓	✓	✓	✓	✓	—
Table of contents from document headings	Heading data	Rendered TOC	Rendered TOC	—	—	—
MDX — components and expressions mixed with Markdown
Recognize JSX, {expressions}, and imports/exports	Limited	Limited	Limited	—	—	—
Application integration
Inspect/transform the parsed document	Events	Tree (AST)	Tree (AST)	Events	Callbacks	Callbacks
Customize generated output	Code hook / events	HTML hooks	HTML hooks	Event transforms	Renderer callbacks	Renderer callbacks

Math support preserves formula notation; an application supplies mathematical typesetting. “Custom tag” wiki links also need application-side routing/rendering. Frontmatter support extracts metadata text, rather than parsing YAML/TOML values. “Heading data” lets an application build a TOC; “Rendered TOC” includes HTML for an inline [[toc]] marker. Trees expose nested document nodes; events/callbacks expose elements in sequence. The three MDX entries cover bounded syntax handling, not a full MDX compiler or JavaScript runtime. Single-tilde strikethrough and subscript compete for the same syntax; enabling subscript gives it priority.

Optional writing syntax

Behavior	Rust ParserOptions	Node option	Default
==marked text== renders as <mark>	highlight	highlight	false
^[inline note] creates a footnote	inline_footnotes	inlineFootnotes	false
Resolve full, collapsed, and shortcut reference links/images	allow_link_refs	allowLinkRefs	true

Inline notes accept inline formatting and work independently of reference footnotes. Disabling reference links preserves definitions as visible Markdown; inline links and images continue to work. Marked text is separate from code syntax highlighting. Rust and Node examples, syntax, and defaults.

The writing extensions remain off by default. Paired measurements found no material slowdown on representative documents with them disabled. Enabling them on documents without matching syntax still costs several percent, and literal marker-heavy inputs cost more. Disabling reference links can skip parsing work, but its benefit depends on the parser profile and changes the output of actual references. See the measured costs and limitations.

Other extension policies

Reference footnotes have document-wide scope, including definitions in lists, block quotes, and Markdown blocks inside MDX components. References can precede their definitions. Code and raw HTML cannot define footnotes.

V2 preserves authored punctuation. Automatic typography belongs in an optional, locale-aware document transform; the former English-oriented parser option has been removed. See the typography decision.

Enable ParserOptions::front_matter to extract a leading --- (YAML) or +++ (TOML) metadata block into document.front_matter. It exposes the format, raw content, and original source spans. The block is omitted from HTML and cannot define Markdown links or footnotes. The option is off in every preset. Frontmatter syntax and Rust example.

Enable ParserOptions::line_comments to omit // note source lines from HTML. Comments may have up to three leading spaces and do not separate paragraphs. Code blocks, raw HTML blocks, and explicitly prefixed lines such as > // text remain literal. The option is off in every preset. Syntax, examples, and source-span behavior.

V1 derives numeric column-width hints from delimiter dash counts. V2 instead supports table IDs/classes, inline captions, and generated colgroup columns whose widths are set in external CSS. Optional header-derived classes such as col-name-netto-preis use the heading slug rules and retain positional classes. Both versions use adjacent pipes for horizontal spans (|| spans two columns). All V2 table extras are opt-in and off in every preset. The table-layout rows describe built-in handling of Markdown tables; raw HTML and custom rendering can provide additional layouts in any engine. Syntax, options, and a runnable CSS example.