[Review] Accessibility: must-level rules, high contrast, contrast pairs, testing

Problem/Motivation

The contract makes forced colors, visible focus and logical properties must-level rules, and delivers a high-contrast variant of the tokens. Once modules build on it, these rules and values set the baseline for a lot of contrib UI.

The full brief, with what we built, screenshots and how to try it in the demo site: https://project.pages.drupalcode.org/design_system_api/reviews/accessibility/

Questions

1. Which accessibility rules must the contract own (reduced motion, target size, …)?
2. Are the high-contrast values and their delivery right?
3. Should contrast between background and foreground tokens be checked, and against which threshold?
4. What do automated checks miss, and is APG + ARIA-AT grading the right basis for pattern tests?
5. Would rules as data help accessibility reviews in contrib?

Short answers help as much as long reviews ("2: no, because …"). Each question in the brief links to the part of the concept it affects.

Proposed resolution

Collect answers here. Settled answers become decisions, change the concept, and are summarized under "What we heard" in the brief.

Remaining tasks

• Review by maintainers of this area.
• Summarize the outcome in the brief.

Proposing changes to the brief

The brief is docs/reviews/accessibility.md in this repository: open a merge request from this issue's fork.