xhtmlmd

A Rust Markdown parser and XHTML renderer.

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

xhtmlmd is largely implemented using AI, except for the tests. The tests are largely adapted from cmark-gfm, PHP Markdown Extra, kramdown, Pandoc, and Mistlefoot. Credit for xhtmlmd really belongs to the authors of these tests, and of the CommonMark docs, which is where the hard work was done.

Implemented syntax

• Core block syntax: paragraphs, ATX/setext headings, thematic breaks, block quotes, ordered/unordered lists, indented code, raw HTML, link reference definitions.
• Tables: GFM/PHP Extra pipe tables with alignment, and Pandoc grid tables with alignment, headerless tables, block cell content, row spans, column spans, and footers.
• GFM: 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 $...$ 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.
• Raw passthrough: a Pandoc-style raw attribute names the format a payload is written for. A fenced code block whose info string is exactly {=name}, or inline code followed immediately by {=name}, renders as <script type="text/x-name"> with the payload entity-escaped. The parser never inspects the name or the payload; each downstream converter documents which names it consumes and ignores the rest.
• Cross-references: Quarto-style bracketed references to identified elements. [@sec-pay] renders as <a data-ref="data-ref" href="#sec-pay"></a>, a symbolic carrier each converter resolves its own way (a number, a Word REF field, a link); [-@sec-pay] marks it bare (data-ref="bare", no prefix word), [Clause @sec-pay] carries override text, and [@sec-a; @sec-b] groups references in a span.refs. A trailing attribute list attaches as usual, e.g. [-@sec-pay]{ref=page}. The bracket group must contain only reference items (prefix text needs a space before the @, and ids are letters, digits, -, _), it loses to explicit link syntax such as [@sec-x](url), and anything that doesn't match stays ordinary text, so [user@host] is untouched. The parser never resolves numbers or checks that targets exist.
• Table captions and figures: a : caption {attrs} line glued directly under a table's last row captions it (attrs apply to the table; Quarto's caption format, glued-only and after-only in ours), and a paragraph that is exactly one image becomes a <figure> with the alt text as <figcaption> (pandoc's implicit figures; the image's id and classes move to the figure).
• Inline footnotes: pandoc-style ^[an inline note], numbered together with [^id] references.
• Smart punctuation (opt-in smart=True): --- and -- to em and en dashes, ... to an ellipsis, and quote curling, in text only; code, math, and raw payloads are untouched.

Attributes

A braced group is an attribute list only when it starts with :, #, ., or a key=value pair. Anything else in braces is ordinary text, so prose like use {braces} freely keeps its content. The marker forms follow Pandoc: {#id .class key="value"}. The colon form follows kramdown: {:note} and {: note} apply the attribute definition named note, and an unknown name in a colon-marked list is ignored while the list itself is still consumed.

ALDs (attribute list definitions) are kramdown's named bundles. {:note: #id .class} on its own line defines note; a reference resolves either as a colon-marked list ({:note}) or as a bare token inside a list already recognized by its markers ({.x note}).

Attribute lists attach to:

• Headings, ATX and setext: # Head {#h}. Headings without an explicit id get a pandoc-style one derived from their text (lowercased, punctuation dropped, spaces to hyphens, -1 suffixes on duplicates); pass auto_ids=False to disable.
• Fenced code: in the info string, python {.numberLines} after the opening fence.
• Fenced divs: in the ::: opener.
• Tables: a trailing list on the glued : caption line applies to the table.
• Link reference definitions: [r]: /url "title" {.external} applies the attributes to every link resolved through that reference.
• Any block, via a standalone IAL line {: ...}. IALs bind by adjacency: glued directly under a block (including the last row of a table) they modify it, glued directly above a block they modify that one, and an isolated IAL with blank lines on both sides is literal text. This is also the only way to attribute a paragraph; a brace group at the end of a paragraph's own text is always literal.
• Inline constructs, when the list follows immediately with no space: spans [x]{.c}, links, images, code spans, emphasis, strong, strikethrough, superscript, subscript, highlight, and math.

Raw HTML blocks take no attribute lists; write attributes in the HTML itself.

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")

Markdown rewriting

rewrite changes recognized Markdown constructs without regenerating the rest of the document. A callback returns None to leave a construct alone, a string to replace the whole construct, or a dict to replace one of its named fields.

This converts inline dollar math to bracket math:

from xhtmlmd import rewrite

def bracket_math(node):
    if node["delimiter"] != "$": return None
    return rf"\({node['tex']}\)"

markdown = rewrite(markdown, {"math_inline": bracket_math}, math="dollars")

An image callback can save a data URL and replace only its destination. The alt text, title, attributes, and original spacing are preserved.

from base64 import b64decode
from pathlib import Path
from xhtmlmd import rewrite

def save_image(node):
    if not node["url"].startswith("data:image/png;base64,"): return None
    path = Path("images/plot.png")
    path.parent.mkdir(parents=True, exist_ok=True)
    path.write_bytes(b64decode(node["url"].split(",", 1)[1]))
    return {"url": path.as_posix()}

markdown = rewrite(markdown, {"image": save_image})

Callbacks run in source order. Their edits are checked first and then applied from the end of the document, so an early replacement cannot invalidate a later source position. Exceptions from callbacks are passed through unchanged.

Every callback node is a dict with these common fields:

• type: callback name, currently image or math_inline.
• source: the exact source text for the construct.
• start, end: half-open character offsets into the original Python string.

An image node has:

• form: currently always inline.
• alt: plain alt text.
• url: the decoded image destination.
• title: decoded title text, or None.

An image callback may return {"url": "new destination"}. Other image fields are read-only. Reference-style images such as ![alt][id] are not callback targets.

A math_inline node has:

• delimiter: $, $$, \(, or \[.
• tex: content without delimiters.
• display: True for $$ and \[, otherwise False.

A math callback may return {"tex": "new TeX"} to preserve the delimiters, or a string to replace the entire construct. Dollar math is recognized only with math="dollars", using the same dollar rules as rendering.

Rewriting is confined to inline-capable prose regions. Inline code, fenced and indented code blocks, raw HTML blocks, block math, link reference definitions, and grid tables are left untouched. Inline images and math inside paragraphs, headings, lists, block quotes, definition bodies, footnotes, and pipe tables are supported.

Callbacks

Python callers can override rendered nodes with callbacks. Each callback receives a node dict and the default XHTML for that node. Return None to keep the default, or return replacement XHTML.

Callback names:

• Blocks: paragraph, heading, block_quote, list, definition_list, code_block, html_block, html_container, thematic_break, table, div, math_block
• Inlines: text, soft_break, hard_break, emph, strong, strike, superscript, subscript, highlight, code, link, image, autolink, abbr, html_inline, math_inline, footnote_ref, span

from fastpylight import highlight
from xhtmlmd import to_xhtml

def highlight_code(node, default_html):
    if node["lang"] != "python": return None
    return highlight(node["text"], node["lang"]) + "\n"

html = to_xhtml(markdown, callbacks={"code_block": highlight_code})

Callbacks can also render bracket math as MathML:

from math_core import LatexToMathML
from xhtmlmd import to_xhtml

mathml = LatexToMathML()

def render_math(node, default_html):
    html = mathml.convert_with_local_state(node["tex"], displaystyle=node["type"] == "math_block")
    return html + ("\n" if node["type"] == "math_block" else "")

html = to_xhtml(markdown, callbacks={"math_inline": render_math, "math_block": render_math})

Block spans

blocks reports where each top-level block sits in the source, so callers can split a document into per-block source slices without regenerating Markdown from a tree. Each dict has type (the callback names above, plus link_ref, abbr_def, attr_def, and footnote_def) and half-open 0-based start/end line indices; code and math blocks also carry their inner text, and fences carry info/lang.

from xhtmlmd import blocks

src = open("input.md").read()
lines = src.split("\n")
for b in blocks(src):
    print(b["type"], "\n".join(lines[b["start"]:b["end"]]))

Command-line usage (the xhtmlmd script is installed with the package):

xhtmlmd input.md > out.xhtml
cat input.md | xhtmlmd --math=dollars

Parsing strategy

The parser uses the two-phase strategy described in the CommonMark parsing-strategy appendix: first build the block tree and collect link reference definitions, then parse raw inline text with the completed reference table. It tracks visual columns and byte offsets for each line and builds blocks with an arena-backed open-container stack. The stack has typed nodes for block quotes, lists, paragraphs/setext candidates, fenced and indented code, raw HTML, table candidates, grid tables, 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. Inputs that can otherwise explode 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 keeps 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 extra protection, not a replacement for sanitizing untrusted rendered HTML.

Raw HTML passthrough means unbalanced source HTML produces an unbalanced fragment, exactly as CommonMark specifies. The opt-in balance option (Options::default().balance is false; --balance on the CLI) restores well-formedness after rendering: unclosed elements are closed at the end of the fragment, stray closing tags are dropped, a closing tag that skips over open elements closes them first, void elements are rewritten to self-closing form, and rawtext elements such as script are copied verbatim to their real close. It deliberately does not apply HTML5 implied-end-tag rules (no <p> auto-close) or rewrite attributes.

Tests

maturin develop && pytest -q

The spec-conformance suite is tests/test_conformance.py: it renders the fixtures under tests/source/ and compares normalized HTML trees. Run just that file with pytest tests/test_conformance.py -v to see per-example ids.