[css-color-4][css-color-5] Allow `<alpha-value>` to be `none` in `<color()>`

[cdoublev]

In 4.3. "Missing"/none Color Components:

All color functions allow any of their components to be specified as none.

The value definition of <color()> is color( <colorspace-params> [ / <alpha-value> ]? ) in CSS Color 4 or color( [from <color>]? <colorspace-params> [ / <alpha-value> ]? ) in CSS Color 5.

Side question: is it voluntary not to export <color()> related sub-productions from CSS Color 5?

[svgeesus]

Sorry, not sure what your actual question is.

CSS Color 4 has a definition for the color() function.

CSS Color 5 expands that definition, adding two things:

1. Relative Color Syntax (the [from <color>]? part)
2. Custom color spaces (via @color-profile

The Color 4 definition is the current, stable implementation target in browsers. Safari implements RCS, and BFO Publisher implements custom color spaces.

[cdoublev]

<colorspace-params>, <custom-params>, etc... are defined with data-dfn-type="dfn" data-noexport, below the definition of <color()>, which is defined with data-link-type="type". The result is that w3c/reffy extracts <color()> but not <colorspace-params>, <custom-params>, etc...

<colorspace-params> is different in CSS Color 4 and 5.

[svgeesus]

Aha, I see! Huh, no reason for data-noexport.

<colorspace-params> is different in CSS Color 4 and 5.

Yes.

[svgeesus]

Might be a bikeshed issue, or might be a difference in the per-spec metadata. See the bikeshed bug I just raised.

[cdoublev]

Thanks! Regarding this:

All color functions allow any of their components to be specified as none.

Do you think that [ / <alpha-value> ]? should be [ / [<alpha-value> | none] ]? in <color()>?

[svgeesus]

Yes, that one is clearly an oversight; all the other functions which take an optional alpha also allow none there.

[svgeesus]

There is now a WPT test for all combinations of <number>, <percentage> and none in color()

[cdoublev]

Thanks for the feed back. Should not none be serialized to none instead of 0?

testColorFunction("sRGB with alpha, all none", "color(srgb none none none / none)", "color(srgb 0 0 0 / 0)");

For syntactic forms which support missing color components, the value none (equivalently NONE, nOnE, etc), shall be serialized in all-lower case as the string "none".

https://drafts.csswg.org/css-color-4/#serializing-color-values

EDIT: fixed in web-platform-tests/wpt@4a49089.