xhtmlmd

A Rust Markdown parser and XHTML renderer.

The parser is deliberately tree-oriented: it preserves structure and attributes needed for XHTML output, but it does not aim to round-trip source text. The dialect is CommonMark/GFM for the core and GFM features, with Pandoc-leaning choices where extension families disagree.

Implemented syntax

• Core block syntax: paragraphs, ATX/setext headings, thematic breaks, block quotes, ordered/unordered lists, indented code, raw HTML, link reference definitions.
• GFM: pipe tables with alignment, task lists, ~~x~~ strikethrough, angle and bare autolinks, plus opt-in tagfiltering.
• Code: backtick/tilde fenced code blocks, info strings, and Pandoc-style code attributes.
• HTML-in-Markdown: block containers opened with markdown="1"; the control attribute is stripped, indented code blocks are disabled inside the container, and fenced code is the code-block syntax there.
• Math: four modes: brackets for \(...\) and \[...\], dollars for those plus $...$ and $$...$$ using Pandoc's non-space/digit dollar rules, on to preserve \(...\) and \[...\] delimiters for client-side renderers such as KaTeX, and off. Brackets mode is the default.
• Attributes and inline spans: Pandoc/kramdown-style {#id .class key="value"}, block IALs {: ...}, span IALs, ALDs such as {:note: #id .class} with references, superscript ^x^, subscript ~x~, and highlight ==x==.
• Definition lists: PHP Markdown Extra/Pandoc-style Term followed by : definition or ~ definition.
• Footnotes: [^id] references to defined [^id]: definitions with indented continuation blocks.
• Abbreviations: *[HTML]: Hyper Text Markup Language definitions render matching text as <abbr>.
• Fenced divs: Pandoc/Quarto/Djot-style ::: containers with attributes or a single class word.

Parsing strategy

The implementation is moving toward the CommonMark parsing architecture: track visual columns and byte offsets for each line, determine block structure with an arena-backed open-container stack, collect link reference definitions, then finalize raw inline text with the completed reference table. The stack has typed nodes for block quotes, lists, paragraphs/setext candidates, fenced and indented code, raw HTML, GFM table candidates, math, footnote definitions, definition lists, fenced divs, and markdown-in-HTML containers. Inlines are scanned into atoms, bracket openers, and delimiter runs; links/images/spans resolve through the bracket stack, while emphasis/strong/strikethrough resolve through the delimiter stack. Potentially explosive constructs have explicit bounds: inline nesting, block/container nesting, link label length, and link parenthesis nesting.

The link parser uses raw reference-label scanning, bounded parenthesis nesting, bounded link labels, URI escaping for rendered href/src attributes, and a plain-text fast path for inputs with no possible inline constructs. This is intended to keep adversarial inputs such as deeply nested brackets, long blockquote runs, repeated ![[](), and unclosed comments in predictable time.

Raw HTML is preserved by default. Supported raw HTML container tags such as div, section, table, svg, math, and custom elements stay open across blank lines until their matching close tag, with same-tag nesting counted; void and self-closing tags do not open balanced containers. Markdown inside raw HTML remains raw unless the open tag that starts the Markdown block uses markdown="1"; this crate does not recursively look for markdown controls inside otherwise-raw HTML. Options::default().tagfilter is false; enabling it applies GFM-style filtering for tags such as script, style, xmp, and textarea. This is compatibility and defense-in-depth, not a replacement for sanitizing untrusted rendered HTML.

Usage

Install via pip to get both the Python API and the native xhtmlmd CLI:

pip install xhtmlmd

The CLI reads Markdown from stdin or from an optional file path and writes an XHTML fragment to stdout:

echo '# Hello' | xhtmlmd
xhtmlmd input.md > out.xhtml
xhtmlmd --math=on input.md > out.xhtml
xhtmlmd --math=dollars input.md > out.xhtml

Python API:

from xhtmlmd import to_xhtml

html = to_xhtml(r"\(x^2\)")
html_for_katex = to_xhtml(r"\(x^2\)", math="on")
html_with_dollars = to_xhtml("$x$", math="dollars")

Rust/source usage:

cargo run --release -- input.md > out.xhtml
cat input.md | cargo run --release -- --math=dollars

Library usage:

use xhtmlmd::{to_xhtml, Options, MathMode};

let mut options = Options::default();
options.math = MathMode::Dollars;
let html = to_xhtml("$x$", &options);

Tests

The tree includes a focused extension fixture in tests/fixtures/, pathological smoke tests in tests/pathological.rs, copied cmark-gfm specs, and a notebook-derived spec from meta/mf.ipynb.

tools/test.sh
cargo test --test conformance -- --nocapture
maturin develop
pytest tests/test_python.py

The spec harness has a per-example timeout and supports XHTML_MD_CONFORMANCE_SECTION, XHTML_MD_CONFORMANCE_EXAMPLE, XHTML_MD_CONFORMANCE_LIMIT, and XHTML_MD_CONFORMANCE_TRACE for narrowing failures.