# Choosing a Processor

Marked can preview the same file with several built-in Markdown processors. Each one makes different tradeoffs between writing workflow (books, blogs, GitHub READMEs) and output control (IDs, classes, metadata). You choose the default in Marked Settings, Processor pane; you can also override the processor per document.

This page summarizes how the four main processors differ. For full syntax details, see the reference pages under Help → Markdown Reference (e.g. MultiMarkdown v5 Specification, Kramdown Specification, CommonMark Specification, Discount GFM Specification).

Contents

- MultiMarkdown (v5)
- Kramdown
- CommonMark (GitHub Flavored Markdown / cmark-gfm)
- Discount
- Quick comparison
- See also

## MultiMarkdown (v5)

Best for: Long-form prose, academic or technical writing, and anything that relies on metadata, citations, and MultiMarkdown-specific features.

Marked ships with MultiMarkdown 5 (see the MultiMarkdown User’s Guide for the upstream documentation).

### Strengths

- Narrative and reference-heavy documents: Footnotes, bibliography/citations, and tables are first-class.
- Metadata: Standard MultiMarkdown metadata blocks (`Key: Value` headers) plus transclusion and other MMD conveniences described in the v5 guide.
- Metadata substitution: Keys from metadata can be inserted in the body with `[%key]`-style replacement so titles, author strings, and similar values stay in sync with the header.
- Tables, images, and cross-references: Aligned with the features documented for MultiMarkdown 5.

### IDs and manual headings

- Heading IDs are normalized in a way that tends to produce lowercase, concatenated slugs (no spaces—words run together).
- For manual header IDs, MultiMarkdown uses the form: `## Headline Text [my-id]` (the bracketed identifier after the heading text).

### When to pick something else

If you need GitHub-flavored task lists and the exact behavior of GitHub’s current parser, prefer CommonMark (GFM). If you need fine-grained HTML classes/IDs on arbitrary elements, consider Kramdown.

## Kramdown

Best for: Documents where you want precise control over HTML output—custom classes, IDs, and attributes, so your CSS can target specific blocks and spans.

The kramdown syntax reference is the authoritative guide.

### Strengths

- Mostly compatible with MultiMarkdown-style habits for everyday Markdown, while adding its own extensions.
- Inline and block attribute lists (IALs): Attach `{: #id .class key="value"}` to paragraphs, headers, code blocks, links, images, and more — ideal for Jekyll-style sites and custom stylesheets.
- Header IDs: kramdown normalizes auto-generated header IDs to lowercase, hyphen-separated words (e.g. `my-section-title`). For manual IDs, use the `{#id}` form after the headline text—e.g. Setext: `My Section {#my-section}` then the underline, or ATX: `# My Section {#my-section}` (see kramdown’s headers for exact placement and IAL rules).
- Definition lists, footnotes, smart typography, math blocks: Rich feature set for publishing pipelines that need more than “plain” Markdown.

### When to pick something else

If you rely on MultiMarkdown-only metadata substitution (`[%key]`) or MMD-specific citation workflows, MultiMarkdown may be a better fit. For README and repo docs that must match GitHub online, CommonMark (GFM) is usually closer.

## CommonMark (GitHub Flavored Markdown / cmark-gfm)

Best for: README files, issue/PR descriptions, and project documentation that should match GitHub’s current Markdown behavior as closely as possible.

Marked uses a GFM-oriented engine (cmark-gfm). The formal spec is the GitHub Flavored Markdown Spec, built on CommonMark.

### Strengths

- Closest match to GitHub: Tables, strikethrough, task list items, fenced code blocks with language tags, and autolinks behave like modern GitHub rendering.
- Unambiguous parsing: CommonMark defines block/inline precedence and list rules precisely—stricter in some edge cases than “classic” Markdown.pl behavior, but more predictable once you learn the rules.
- Practical for wrapped text: Paragraph and list rules are designed to behave well with hard-wrapped prose (see the spec’s sections on lazy continuations and lists).

### Header IDs

Auto-generated heading anchors are typically lowercase and hyphen-separated, consistent with common GitHub-style slugging.

### When to pick something else

GFM does not replicate MultiMarkdown metadata, kramdown IALs, or MMD citation workflows. For books, theses, or heavy metadata, use MultiMarkdown or Kramdown as appropriate.

## Discount

Best for: A fast, C-based processor that tracks classic Markdown and an older GitHub-flavored feature set—useful when you want behavior closer to original Markdown plus tables, footnotes, and related extensions without the full CommonMark rule book.

Project home: Discount.

### Strengths

- PHP Markdown Extra-style tables and many extensions (footnotes, fenced code when enabled, etc. — see Marked’s Discount GFM Specification for what Marked enables).
- Optional “GitHub” extras in upstream Discount (e.g. checkbox lists when built with the right flags); Marked documents the combination it ships in the Discount spec page.
- SmartyPants-style typography and other conveniences described on the Discount site (though all included processors actually provide typography features).
- Philosophically close to John Gruber’s Markdown plus practical extensions, rather than the full CommonMark test suite.

### When to pick something else

For pixel-perfect parity with today’s github.com, prefer CommonMark (GFM). For MultiMarkdown metadata and citations, use MultiMarkdown.

## Quick comparison

Concern | MultiMarkdown | Kramdown | CommonMark (GFM) | Discount
--- | --- | --- | --- | ---
Primary use | Prose, papers, books | Styled HTML, Jekyll-like sites | READMEs, GitHub-like docs | Classic MD + extensions
Citations / MMD metadata | Strong | Via different syntax | No | No
Manual heading ID style | `## Title [id]` | `## Title {: #id }` (IAL) | Spec / GitHub slug rules | None
Auto heading IDs | Lowercase concatenated | Lowercase hyphenated | Lowercase hyphenated | Lowercase-hyphenated
Extra attributes (classes/ids) | Limited MMD mechanisms | IALs — very strong | Limited | Limited

## See also

- Settings: Processor — default processor and related options
- Markdown Dingus — try processors side by side in Marked
- Custom Processor — plug in your own toolchain when needed

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