[css-color-5] Tests for resolved alpha(from currentcolor ...) values have incorrect serialization #60611

See the tests here in alpha-color-computed.html. An example test case:

test_currentcolor_alpha("alpha(from currentcolor / 0.5)", "color(srgb 1 0 0 / 0.5)", "red");

This test gives the element color: red and then specifies background-color: alpha(from currentcolor / 0.5). It then asserts a resolved value for background-color of color(srgb 1 0 0 / 0.5). But my reading of the Level 4 and 5 specifications is that the actual resolved value should be rgba(255, 0, 0, 0.5). Here's how I'm getting that:

Specified Value

The specified value is just alpha(from currentcolor / 0.5) on the background-color property.

Computed Value

From Level 5 § 10.2. Resolving Relative Color Syntax, it says:

Otherwise (if currentColor was used in the function), the computed value is the Relative Color Syntax function with the origin parameter resolved according to CSS Color 4 §  15. Resolving Values, thus preserving inheritance into child elements.

And since currentcolor computes to itself, then the the computed value of this color is the same thing, the relative color function alpha(from currentcolor / 0.5).

Used/Resolved Value

To find the used/resolved value, it needs to resolve the currentcolor. In Level 4 § 15.5. Resolving other colors, it says:

In the color property, the used value of currentcolor is the resolved inherited value. In any other property, its used value is the used value of the color property on the same element.

So the used/resolved value of currentcolor in background-color is just the resolved value of the color property, which is the sRGB color value that serializes to rgb(255, 0, 0). With that, we can resolve the function per Level 5 § 10.2. Resolving Relative Color Syntax:

If all parameters resolve to the corresponding colors in their respective color spaces, the computed value is the absolute value, in the specified RCS color space, resolved according to CSS Color 4 §  15. Resolving Values.

For the alpha() function, the absolute value and color space are defined per Level 5 § 4.10. Relative Alpha Colors as:

The color components of the origin color are unchanged, the alpha component is modified or replaced. The result of this function is in the color space of the origin color.

This means that the used/resolved value of alpha(from currentcolor / 0.5) is the origin color (resolved to the sRGB color represented by rgb(255, 0, 0)) in its original color space (sRGB) with the modified alpha component (0.5). As an sRGB color value, this should serialize as rgba(255, 0, 0, 0.5).

I believe that according to Level 4 § 15.4. Resolving values of the color() function, the only way to serialize a resolved color value to color(srgb ...) would be if the specified value itself was color(srgb ...). That's not the case in these alpha(from currentcolor ...) teset cases, and therefore the color() function should not be appearing in the serialization.

One note on the specification is that Level 5 § 10.2. Resolving Relative Color Syntax only refers to the "computed value" stage when I think it should be clearer about computed vs. used/resolved.