Add the lack of proper gamut mapping in OKLch and Oklab. The browser support is thus only parial.

[pygy]

Summary

The current browser implementation of gamut mapping is buggy and not spec compliant. This has accessibility ramifications (the spec ensures that color contrast is preserved, even if a color is beyond what a user can display, but browser don't respect that invariant in their implementation).

Test results and supporting details

Visit https://vasilis.nl/dingen/bugs/oklch.html

The squares should be white and black per spec, they aren't in any browser.

• https://github.com/w3c/csswg-drafts/issues/9449
• https://bugs.webkit.org/show_bug.cgi?id=255939
• https://bugzilla.mozilla.org/show_bug.cgi?id=1847421
• https://issues.chromium.org/issues/40909194
• https://github.com/w3c/csswg-drafts/issues/7610
• https://github.com/w3c/csswg-drafts/issues/7653

Related issues

This fixes #26838

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