[css-color] We should consider addressing different use cases contemplated for contrast-color() with separate CSS functions

[hober]

There are a number of different use cases & requirements for "please give me a color that contrasts with this/these other color(s)" functions, but given the differences between these use cases, perhaps they'd best be addressed with additional color functions.

1. "Given this color, please compute a different color for me that contrasts with it." Variations of this include distinguishing between "this is intended for foreground text v. a background under text" and some threshold of sufficient contrast (WCAG, for instance, has 3 different thresholds for different situations).
2. "Given a color and a set of colors, tell me which color from that set maximally contrasts" (same fore/background and threshold variations as in (1), but the key difference here is that the author provides a set of colors and there's a guarantee that the returned color will be from that set)
3. "Given a pair of colors, tweak one or both of them the minimum amount to get the contrast 'good enough'" (same fore/background and threshold variations)
4. Contrasting color over an unknown background (e.g. an image)

Two relevant personas:

1. Maintainer of existing, small/medium (not Google-scale) website(s) who want to improve legibility/a11y without messing with the existing design much
2. Designer at company with very particular and exact brand guidelines who can only use colors from a pre-defined set

@cookiecrook @fantasai

[hober]

Note about WCAG thresholds: they IIRC have different ones for body text, for headline text, and for things like borders.

[hober]

This is related to #11534

[cookiecrook]

WCAG thresholds:

• 3:1 large scale (headline) text, and non-text contrast (AA)
• 4.5:1 minimum contrast for standard text and [debated] text-like iconography (e.g. button icons) (AA)
• 7:1 enhanced contrast when requested by the user through some unspecified means (AAA, optional but desirable)