[css-color-6] It should be possible to specify the contrast algorithm for color-contrast()

[LeaVerou]

Since we know that WCAG 2.1 has huge known problems and WCAG 3.0 isn't quite ready yet, we should not ship a color-contrast() that mandates use of a broken algorithm.
Instead, the algorithm should be explicitly specified with either syntax.

Since passing levels depend on the algorithm and 4.5 in WCAG 2.1 may not be the same as 4.5 in another algorithm, I'd suggest it's specified via functions, e.g. color-contrast(white vs yellow, blue to wcag2(4.5)) or color-contrast(white vs yellow, blue to wcag2(AA))

(Issue filed following breakout discussions between @svgeesus, @fantasai, @argyleink and myself)

[SebastianZ]

Question is also what to do if the specified algorithm (or maybe even the contrast ratio) are not supported by the UA. Should it be invalid at computed value time or should the UA fall back to its default algorithm or something else?

Sebastian

[argyleink]

works for me.
i'm only a tiny bit curious why it's a function, not really how other css functions work, but i mean it matches the mental model so thats nice.

[LeaVerou]

Question is also what to do if the specified algorithm (or maybe even the contrast ratio) are not supported by the UA. Should it be invalid at computed value time or should the UA fall back to its default algorithm or something else?

No reason to be invalid at computed value time, it should be invalid at parse time so it can be detected by @supports

i'm only a tiny bit curious why it's a function

Because levels don't mean anything outside their algorithm, so they need to be associated with that. If levels were just keywords we could hyphenate (e.g. wcag-aa), but since they can also be <number>, a function is the only way to go.

[Myndex]

HI Lea @LeaVerou

...levels don't mean anything outside their algorithm, .... (e.g. wcag-aa),..

Not only that, but: AA encompasses two discrete ratios, 3:1 and 4.5:1, and AAA encompasses three distinct ratios, those of AA, and the additional 7:1.

And the 3:1, 4.5:1, and 7:1 as calculated by WCAG 2 math are different than every other contrast math, and the actual results skew differently relative to the overall total luminances being tested as does every other contrast math or method, ever...

And I know you know this but for anyone reading along at home, among the reasons is that there is no such thing as an absolute contrast: Contrast is not real, just as color is not real, it is a context sensitive perception and about as hard to nail down as grasping a wet bar of soap in the middle of a typhoon while being chased by hungry wild geese.

[svgeesus]

Not only that, but: AA encompasses two discrete ratios, 3:1 and 4.5:1, and AAA encompasses three distinct ratios, those of AA, and the additional 7:1.

Yes, we know; in CSS we have separate keywords for those:

The keyword AA is equivalent to 4.5, AA-large is equivalent to 3, AAA is equivalent to 7, and AAA-large is equivalent to 4.5 .

And the 3:1, 4.5:1, and 7:1 as calculated by WCAG 2 math are different than every other contrast math,

Yes, that is what "Because levels don't mean anything outside their algorithm" is saying.