Intent to ship: colorspace and alpha attributes for `<input type=color>`

[Kagami Rosylight]

Summary: The new colorspace and alpha attributes controls the output by color picking, e.g using the new color() syntax in the designated colorspace.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1919718

Specification: https://html.spec.whatwg.org/#color-state-(type=color)

Standards Body: WHATWG

Platform coverage: All
Preference: dom.forms.colorspace_and_alpha.enabled

Link to standards-positions discussion: https://github.com/mozilla/standards-positions/issues/1230

Other browsers: 

• Blink: https://issues.chromium.org/issues/368059226
• WebKit: Shipped in Safari 18.4

web-platform-tests: https://wpt.fyi/results/html/semantics/forms/the-input-element/color.window.html?label=master&label=experimental&aligned&q=color.window.html

This is happening in parallel with the color picker change, the picker will not have the colorspace and alpha support until we fix all known UX issues and decide to ship the new color picker, with the intent to prototype here. If display-p3 is requested by colorspace attribute, the picker will remain in sRGB for now and then be converted to display-p3 in the value string. Requesting alpha will only affect the output format without actual alpha selection for now, unless you are in Nightly.

We also changed the style rule for ::-moz-color-swatch to represent alpha channel in the color button. We do not know of any web usage of this legacy proprietary pseudo element in a way to affect the color, but if you do or are affected, please feel free to report.