[a11y][List] Assess WCAG 2.2 conformance #49436

Context

Wave 4, item 7. Tracking issue: #49260.

Include ListItem, ListItemText, ListItemIcon, ListItemAvatar, ListItemSecondaryAction and ListSubheader. Focus on list semantics (1.3.1), the association of ListSubheader with its group, and secondary actions in the reading and focus order (1.3.2, 2.4.3).

Scope

1. Add axe tests and fixtures in test/regressions/a11y/fixtures/.
2. Write List/accessibility.md.
3. Enroll it in the CSS-dependent suite (1.4.10, 1.4.4, 1.4.12).
4. Run pnpm a11y:scorecard.

Manual tests

Write each check in the report as numbered Manual testing steps, as in Button/accessibility.md. pnpm a11y:scorecard then adds them to manual-testing.md. For screen-reader steps, use NVDA with Chrome and VoiceOver with Safari.

• 1.3.1 — With a screen reader, go to a List. The screen reader announces "list" and the item count.
• 1.3.1 — Record how the screen reader relates a ListSubheader to the items under it.
• 1.3.2, 2.4.3 — In a list with secondary actions, the reading order and the focus order go from the primary content to the secondary action of the same item.
• 2.4.11 — With a sticky ListSubheader, press Tab through the items. The subheader does not fully hide a focused element.
• 1.4.3 — With a contrast checker, measure the primary and the secondary text of ListItemText. Each is at least 4.5:1.

Done when

• List has a report and is in the scorecard.
• Each manual test in this issue has steps and a result in the report.

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