[css-color-5] Mixing alpha

[kbabbitt]

In #4915 the working group resolved that in forced colors mode, the color-mix function would be used for background-color, in order to apply the computed color's alpha to the appropriate system color.

At the moment color-mix does not specify a way to mix the alpha channel. Noting that a is currently used for the like-named channel in lab colorspace, and alpha is (at least theoretically) independent of colorspace, my suggestion is to add alpha to the production for color-adjuster:

<dfn>color-adjuster</dfn> = <<rgb-adjuster>> | <<hsl-adjuster>> | <<hwb-adjuster>>
    | <<xyz-adjuster>> | <<lab-adjuster>> | <<lch-adjuster>> | alpha

[svgeesus]

Implemented premultiplied alpha for color.range() in color.js:

[svgeesus]

Missing premultiplication is now fixed, see Interpolating with alpha - which applies to all color interpolation, not just for color-mix()

[svgeesus]

As suggested, alpha was added to color-adjuster

[svgeesus]

@kbabbitt do these changes resolve your comment?

[kbabbitt]

@svgeesus Thanks for the updates! Yes, these changes resolve my comment.