The primary guidance for web accessibility on a global basis are the Web Content Accessibility Guidelines, commonly referred to as WCAG. WordPress requires that all content meet WCAG guidelines, but also pushes to provide accessibility beyond that minimum whenever possible.
WordPress Accessibility Coding Standard
All new or updated code released in WordPress must conform with the WCAG 2.2 guidelines at level AA.
The documentation accessibility and WCAG is now published on the WP Accessibility Knowledge Base with:
Note: The documentation in the WP Accessibility Knowledge Base is in development and will be moved to develop.wordpress.org when it’s finished.