[css-color-4] Does CSS Color normatively require that HTML elements use/match system-colors in the UA stylesheet?

[weinig]

The WPT test css/css-color/system-color-consistency.html seems to require that the UA stylesheet use (or at least match at computed value time) the system-colors defined in CSS Color 4.

      test_computed_value('color', 'ButtonBorder', style.getPropertyValue('border-color'), 'resolves to the same color as the border-color of a ' + element.name);

I can't tell if this is a real requirement of the spec, or just something that the spec is suggesting. If it is a real requirement, it would be helpful if the spec was updated to explicitly specify which HTML elements and properties this applies to.

[svgeesus]

For systems that do not have a particular system UI concept, the specified value should be mapped to the most closely related system color value that exists

I think that makes it hard to test, and te descriptions are deliberately vague and non-prescriptive. The actual WPT is fairly conservative, like testing for the color of the canvas.

I think this makes more sense to test together with forced color mode, tbh.

[svgeesus]

To answer the specific question as asked: I think that WPT is poor because "most closely related value" is not testable. The spec is giving guidance, not making a hard requirement.

Closing as question answered and no change to the spec, but feel free to point to this issue as justification when deleting or trimming down that test.