Since accessibility is so critical, it's nice to read that it's "built-in" but are there some more details? I'm interested in how the new Codex will enable MediaWiki and other projects to be fully compliant with WACG and also how developers or testers should use tools to verify accessibility in Codex.
Topic on Talk:Codex
Answering my own question, I see the README states "Web accessibility compliant (Web Content Accessibility Guidelines 2.1 level AA)"
Digging further, there is info in the project sources (docs) at packages/codex-docs/docs/style-guide/accessibility.md which includes reference to wikipedia:Wikipedia:Manual_of_Style/Accessibility Wikipedia's Manual of Style (that goes into great depth about Content Accessibility); plus links to the Accessibility guide for developers that describes techniques, guidelines and some tools for developers.
It's only a suggestion since I'm not that familiar with the sources, the tools (such as automated cross-browser testing), and project contributors' methods but perhaps there should be some guideline or specific process added to e.g. https://doc.wikimedia.org/codex/latest/contributing/contributing-code/testing-components.html that says something like "Open WAVE or your Firefox Accessibility Inspector to confirm WCAG 2.1 level AA compliance".