Manual verification.
Tab through the entire page, use Shift+Tab, Enter, Space, Arrow keys, Home, End and Escape. Confirm spoken labels with a real screen reader, then repeat at browser zoom and on a touch device.
Labels and focus
Semantic status
Success status text should remain understandable without colour alone.
Danger alert contains meaningful text, not colour alone.
Keyboard interaction
Keyboard users should be able to open and close this region.
This region should be hidden from assistive technology while closed.
The trigger should expose this description.
Check that focus enters each overlay, stays inside with Tab, Escape closes it and focus returns to the opener.
Operating-system accessibility modes
High contrast
Enable your OS/browser high-contrast or increased-contrast setting. FG UI contains prefers-contrast and forced-colors rules.
200% zoom
Zoom the browser to 200% and check that content remains usable without two-dimensional scrolling for ordinary layouts.
Screen reader
Core semantics and ARIA state are automatically checked, but final release still requires a real screen-reader pass such as NVDA, VoiceOver or another supported assistive technology.