Intent to Prototype: alpha() relative color function

[Sajid Anwar]

Summary:
The alpha() relative color function allows creating a new color by only modifying the alpha component of an origin color, while retaining the original color space and color components. This provides a simpler way for authors to create translucent versions of colors without needing to know the original color space or to duplicate color components using a different relative color function.

We intend to enable this function in Firefox Nightly builds behind the layout.css.alpha-color-function.enabled pref.

Bug: 
Bug 2042646 - Implement alpha() function

Specification: 
https://drafts.csswg.org/css-color-5/#relative-alpha

Standards Body: 
CSSWG / W3C

Platform Coverage: 
All

Preference:
layout.css.alpha-color-function.enabled — Bug 2047437 to enable in Nightly

DevTools Bug: 
N/A

Extensions Bug: 
N/A

Use Counter: 
N/A

Standards-Positions Discussion: 
https://github.com/mozilla/standards-positions/issues/1396 — Positive

Other Browsers:
- Blink: Estimated to ship in Chrome 150 (Intent to Ship)
- WebKit: Implementation in progress (https://github.com/WebKit/WebKit/pull/66126)

web-platform-tests:
https://wpt.fyi/results/css/css-color/parsing/

Note: There are open questions regarding serialization of alpha() color values that may affect some existing test cases. See the following issues for details:
- https://github.com/w3c/csswg-drafts/issues/13992
- https://github.com/w3c/csswg-drafts/issues/13994
- https://github.com/web-platform-tests/wpt/issues/60611

[Nicolas Chevobbe]

That's great :)

Note that I was playing with the current version in Nightly and I found a couple issues for DevTools:

- Bug 2047869 - Picker for alpha() result color does not use the right color for color contrast information

- Bug 2047863 - Don't show contrast information for base color in alpha()