a11y-wasm

The rule set as WebAssembly: the same rules that run at build time and in CI run with it in the opened page. The package translates, it does not evaluate.

Status

Crate 0.14.0 on crates.io, extracted from liveaudit/packages/core (liveaudit-core, was publish = false). The WASM artifact is published as the npm package @casoon/a11y-wasm; the npm release runs under its own tag (a11y-wasm-npm-vX.Y.Z, see Releases) and may lag behind the crate. The first consumer is liveaudit.

Structure

Which rules run depends on which columns the collector supplies: structure only, additionally semantics, additionally rendering. If a layer is missing, the rule falls back to NotRun — not to “passed”.

Building

wasm-pack has no --profile and would build with [profile.release] (opt-level 3). Here size takes priority over speed, so packages/a11y-wasm/build.mjs builds with [profile.wasm-release] and calls wasm-bindgen itself, followed by wasm-opt -Oz.

The CLI version of wasm-bindgen must match the crate version, otherwise the step aborts with a schema error.

Public surface

Arena (with set_scopes), SemanticArena, RenderArena, RenderingColumns, LayoutColumns, ProbeColumns and the wasm-bindgen entry points for a scan. Rust consumers can also use the crate without WASM as an rlib.

new Scan(...) takes the eleven column arrays of the collector and validates them (equal lengths, attr_start of node count + 1 entries, monotonic and within the attribute columns, parents before children, dictionary indices and blob ranges in bounds). A mismatch makes the constructor throw an Error naming the column; it does not trap the module. run() is typed: the generated .d.ts carries Report, Finding, RuleRun, Summary and the enums, hand-written to mirror the serialisation of a11y-report and checked against it by a test.

Optional layers on top of the arena, each its own call before run(): withRendering(color, background, font_size_px, font_weight, flags) for the computed styles (Tier 3, without it the contrast rules do not run) and withLayout(flags, order, min_width_px, bounds) for the heuristic rules; plus the two groups below.

Tree scopes (shadow DOM)

scan.withScopes(scopes: Uint32Array) takes one tree scope per node, parallel to the node columns: 0 is the document, every other number a shadow root (chosen by the collector, unique per document, below the node count). A node inserted through a <slot> carries the scope of the outer tree, not that of the shadow root it hangs under in the flat tree. a11y-dom resolves IDs per scope (aria-labelledby, aria-describedby, label[for], headers, duplicate IDs): a reference only hits targets of its own scope, and the same ID in two scopes is no duplicate. Without the call every node is in the document. A mismatch (length, scopes[0] != 0, a value at or above the node count) makes the call throw an Error naming the column. Call it before run().

Probe columns (cargo feature probes, default)

Five rules read measurements that only the page can supply: the Rendering methods frame_reachable, box_transform, paste_probe, focus_cover and text_spacing_clip of a11y-dom (their doc comments define what is measured). Each has its own method on Scan. A column that is not supplied is “not measured” (None) and its rule is recorded as NotRun with a reason; a supplied column counts as measured. All columns are sparse: nodes lists arena indices in strictly ascending order, every value array is parallel to it. Nodes a column does not list are not measured. They only take effect together with withRendering, like withLayout.

Method	Rule	Value arrays
withFrames(nodes: Uint32Array, reachable: Uint8Array)	frames/cross-origin	reachable: 1 readable, 0 not readable
withTransforms(nodes: Uint32Array, transform: string[], rotate: string[])	orientation/root-rotated	computed transform and rotate of html/body as serialised by the browser; "" = field not measured
withPasteProbes(nodes: Uint32Array, flags: Uint8Array)	auth/paste-blocked	flags: bit 0 event cancelled, bit 1 probe text inserted
withFocusCover(nodes: Uint32Array, points: Uint32Array, covered: Uint32Array, blocker: Int32Array)	focus/obscured, focus/obscured-enhanced	sample points, covered points (at most points), node index of the topmost blocker or -1
withTextSpacing(nodes: Uint32Array, flags: Uint8Array)	text-spacing/clipped	flags: bit 0 clipped before, bit 1 clipped after the text-spacing override

Calling a method again replaces its column. Validation throws an Error (invalid <column> columns: …) for unequal lengths, node indices that are not strictly ascending or outside the arena, flag bits outside the table, covered > points and a blocker outside the arena; it does not trap the module. The TypeScript signatures come from the generated .d.ts.

The feature is on by default; --no-default-features builds the lean variant without the methods. The columns pull the bodies of the five rules into the artifact, about 9 KB gzip. Budget decision of 2026-11-09 (liveaudit#16): the gzip budget rises from 170 KB to 180 KB (earlier steps: 110, 130, 150, 170 KB) and the columns ship in the default build (measured 183,122 bytes gzip, 178.8 KiB).

Limits

No collection: the package opens no page and reads no DOM. Anyone who collects the tree incorrectly gets incorrect findings — responsibility for shadow DOM, iframes and the timing of the snapshot lies with the collector.