[css-color-6] Meta-issue: `contrast-color()` issues

This is a meta-issue for the various color-contrast() issues. Last updated Feb 16th, 2024.

Level 5 contrast-color() (MVP)

• #9166
• #9970
• #8539

Level 6 contrast-color()

A summary of the current situation is as follows:

• Biggest roadblock: There is currently no problem-free contrast algorithm we could use as a default.
  • WCAG 2.1 algorithm, despite very wide usage, is severely flawed, even for only grayscale colors, or only picking between black and white. False positives/negatives up to 40% (!).
  • APCA which has been advertised as a successor has issues too, including a patent disclosure (not an exclusion), which makes baking it in CSS tricky.
  • There are other contrast algorithms (Weber, Michelson etc) but do not generally produce good results.
  • Relevant issues:
    • #7356
    • #7361
    • #7357
• Satisfy more of designers' use cases
  • #5153
  • #7360
  • #7358
  • https://github.com/w3c/csswg-drafts/issues/8457
• Syntax
  • #7354
  • #7359
  • #7556
  • #7557
  • #7937
  • #7954
• Shortcuts and improvements over syntax
  • #7345
  • https://github.com/w3c/csswg-drafts/issues/6319

(Please refrain from discussing the sub-issues here, but in the relevant linked issues.)

No source files or tests are named. Start by reviewing the CSS Color 6 Level 5 and Level 6 sections and the linked issues on contrast algorithms, use cases, and syntax. This meta-issue is complete only when the underlying contrast-color decisions are resolved across the listed sub-issues.