[css-properties-values-api] Computed values for <color>

[jyc]

Section 2.4 of the spec says:

For <custom-ident>, ident, <color>, <image>, <url>, <integer>, <angle>, <time>, <resolution>, <transform-function> or "*" values, the computed value is identical to the specified value.

But what if a color value is currentColor, or a color name like red?

css-color level 4 says that the computed value of the color property is an RGBA color.

Should the computed value of a variable with type <color> be an RGBA color as well?

[bfgeek]

@tabatkins and I ran into currentColor in #133 as well.

If I understood correctly currentColor now specified to be an actual color at used-value time.
css-color level 4 might be inconsistent with itself? (see: https://drafts.csswg.org/css-color-4/#currentcolor-color )

[tabatkins]

The computed value of color is always an rgba color; that has nothing to do with currentcolor.

currentcolor, when used in any other <color> context, is indeed preserved at computed-value time, and only turned into rgba at used-value time.

[jyc]

Then is currentColor considered "computationally idempotent" in terms of the initialValue parameter?

[shans]

There's a problem here.

background: currentcolor;
will repaint when color changes from red to green; but

background: paint(foo); --background-color: currentcolor; ... registerPaint(“foo”, …, [“--background-color”]);
will not repaint when color changes, because currentcolor is computationally idempotent.

You could just add color to the list of sensitivities in registerPaint, but that seems like a bit of a hack given that currentcolor is valid all over the shop. Would it make sense to special-case currentcolor instead? This would require --background-color to be registered as a color property.

[tabatkins]

Yes, I think we should bite the bullet and special-case currentcolor. It's used as the initial value of a bunch of properties already (border-color, text-decoration-color, etc), and there's definitely custom property use-cases that want the same thing.

[shans]

(Note this also applies for line-height and custom layout)

[tabatkins]

My current definition actually captures this correctly:

A property value is computationally independent
if it can be converted into a computed value
using only the value of the property on the element,
and "global" information that cannot be changed by CSS.

"currentcolor" as a specified value becomes "currentcolor" as a computed value, no extra information necessary (or it becomes #000 for 'color', same deal). It's not until used-value time that extra information is pulled in to resolve it into something different.