html-conform-wasm

html-conform as WebAssembly: check(html) returns the findings in the shared model. The package translates, it does not check.

Status

Crate html-conform-wasm 0.1.0 and npm package @casoon/html-conform-wasm, not yet published. The npm release runs under its own tag (html-conform-wasm-npm-vX.Y.Z, see Releases). Use cases are editor integrations, browser extensions, Cloudflare Workers and LiveAudit.

Structure

Building

packages/html-conform-wasm/build.mjs builds like the one of a11y-wasm: [profile.wasm-release], the wasm-bindgen CLI in the version of Cargo.lock, then wasm-opt -Oz. At the end it checks the size budget and aborts when it is exceeded. The HTML5 schema is not compiled at load time: the artefact crates/html-conform/prebuilt/html5.rngc (barrierlab#72) is embedded and read at the first call.

Size and start-up

Measured on 2026-11-09 (barrierlab#73), Node 24, Apple silicon, wasm-opt 133:

	Value
WASM, raw	1,988 KiB (2,035,930 bytes)
WASM, gzip -9	747.2 KiB (765,111 bytes)
WASM, brotli	about 563 KiB
npm tarball	793.6 kB
Budget (build.mjs)	760 KiB gzip
initSync (instantiate)	2 to 5 ms
first check (small document)	65 to 91 ms
later check calls (same document)	7 to 15 ms

Most of the size is data: the prebuilt schema is 217 KiB raw, the rest is the parser, the RELAX NG validator, the XPath engine and the Schematron rules. The first call pays for loading the schema and the rule set, later calls reuse them.

Public surface

check(html: string): Report and the TypeScript types of the report, hand-written to mirror the serialisation of a11y-report (a test compares the fields). Finding.location.position carries line, column (both from 1) and the byte offset. Severity mapping as in CheckReport::to_report(): an error is fail with medium weight, a warning review, an info untested.

Not run is visible

• Content the checker does not validate (CSS in <style>, CSP hash sources) appears in inputs as document_incomplete.
• A document nested deeper than 512 levels yields no findings and a rule_runs entry html-conform with not_run: "errored" and a reason.
• check throws an Error if the checker cannot be set up.

Dependencies

Downward: html-conform and a11y-report. Upward: nobody inside this repository.

Limits

The check is compared differentially against vnu, not identical to it; which classes are missing is listed in the html-conform documentation. The package does not fetch a page and does not parse anything but the string it is given. It is single-threaded: a long document blocks the thread it runs on, so run it in a worker for large input.