[css-color] handling of powerless and missing components

Description

Browsers do not handle powerless and missing components correctly in interpolation or relative color syntax.

This covers both color-mix, relative color syntax and gradients.

This is important to fix as soon as possible because there is no way to detect support for the correct implementation through @supports.

Specification

https://drafts.csswg.org/css-color-4/, https://drafts.csswg.org/css-color-5/

web-feature

color-mix
color-mix-variadic
gradient-interpolation
relative-color

Test Links

https://wpt.fyi/results/css/css-color/parsing/color-computed-none.html?label=master&max-count=1
https://wpt.fyi/results/css/css-color/parsing/color-computed-powerless.html?label=master&max-count=1

Some tests are also present in, but the core aspects around none and powerless components are tested in the files linked above
https://wpt.fyi/results/css/css-color/parsing/color-computed-relative-color.html?label=master&max-count=1
https://wpt.fyi/results/css/css-color/parsing/color-computed-color-mix-function.html?label=master&max-count=1

Additional Signals

No response

Start with the CSS Color 4 and 5 specifications and the linked WPT files: color-computed-none.html, color-computed-powerless.html, color-computed-relative-color.html, and color-computed-color-mix-function.html. Run the relevant CSS Color tests and compare their coverage of color-mix, relative color syntax, gradients, missing components, and powerless components. Done means the affected behaviors are correctly covered and the tests pass.