[test][linear progress] Add axe tests and WCAG conformance report

[michelengelen]

Adds axe coverage and the WCAG 2.2 Level A/AA conformance report for LinearProgress. Also un-excludes the progress slug from the demo bundle so axe can reach these demos; screenshots stay disabled slug-wide.

Result	Count
✅ Supports	8
⚠️ Partially Supports	3
❌ Does Not Support	0
➖ Not Applicable	44

Known gaps: fill-versus-track below 3:1 for most palette colours in light mode (1.4.11), indefinite animation with no pause control (2.2.2), and value changes not announced (4.1.3).

Report: LinearProgress/accessibility.md

Important

How to review this PR

Layer 9 of 14 in a stacked series (#48915 → #48926). Its branch is built on a11y/stack/toggle-button (#48921). GitHub cannot chain PR bases across a fork, so every PR in the series targets master instead.

Review only the last commit — 2db86bf. Everything above it belongs to the layers below, and disappears as those merge.

Original work by @mj12albert. Supersedes #48735, rebased onto current master.

Part of the WCAG conformance effort (#14187).

[code-infra-dashboard]

Deploy preview

• docs/data/material/components/progress/LinearProgressA11yColorMatrix.js

  
  

Bundle size

Bundle	Parsed size	Gzip size
@mui/material	0B(0.00%)	0B(0.00%)
@mui/lab	0B(0.00%)	0B(0.00%)
@mui/private-theming	0B(0.00%)	0B(0.00%)
@mui/system	0B(0.00%)	0B(0.00%)
@mui/utils	0B(0.00%)	0B(0.00%)

Details of bundle changes

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Check out the code infra dashboard for more information about this PR.

[michelengelen]

PR review

The risky move here — re-bundling the progress slug that was excluded as "Flaky" — is handled correctly: screenshots stay disabled slug-wide via SCREENSHOT_RULES, only axe runs on the enrolled demos, and demoMeta.test.ts pins both the enrolment and the screenshot opt-out. Nothing merge-blocking. One misplaced comment.

Docs (1)

1. 🟡 Checkbox rationale comment attached to the progress rule

Location: test/regressions/demoMeta.ts:290

// FormControlLabelPosition is not enrolled: its only axe finding is an aria-label on a
// role-less FormGroup div (aria-prohibited-attr), a demo quirk unrelated to Checkbox.
{
  test: `docs/data/material/components/progress/{${LINEARPROGRESS_A11Y_DEMOS.join(',')}}`,

The comment explains a checkboxes-page decision (a FormControlLabelPosition demo exists on both the checkboxes and switches pages) but sits directly above the LinearProgress rule. A reader scanning the progress block will attach it to the wrong component.

Failure scenario: the comment reads as if a progress demo were excluded; the actual checkbox exclusion loses its documentation.

Fix: move the comment up next to the checkbox rules — it belongs in #48918's scope.

Verdict

Approve after nits - the flaky-slug tradeoff is solved properly; only a comment is in the wrong place.

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