Accessibility guides

This first batch publishes 2 of the 4 upstream Accessibility guides; the rest are converted and will be released in later batches.

• Accessible Error Announcement — Synchronize programmatic accessibility states (like aria-invalid) with the visual :user-invalid state to ensure screen reader users receive error feedback only after interaction, mirroring the visual experience.
• Respect Operating System Text Scale — Respect the operating system’s text scale preference for a page so font sizes, layout, and rem-based measurements follow the user’s accessibility settings.