A Playwright job on the perf guard's production build runs axe-core's
WCAG 2.2 A/AA rules on the mail list, reading pane, composer, calendar,
contacts and Settings > Accessibility, and fails when a screen has more
serious or critical violations than scripts/a11y-guard/budget.json
allows (all zero today). Two more checks cover what axe can't see: the
keyboard focus outline stays visible in Windows contrast themes, and
single-key shortcuts don't act on the mail behind an open dialog.
axe-core is vendored next to the spec (MPL-2.0, see NOTICE), so the
check adds no dependency. `A11Y_GUARD_UPDATE=1 npm run test:a11y-guard`
rewrites the budget.
