[css-transitions][css-color] Animating currentColor

[frivoal]

The definition of color interpolation isn't explicit about what should happen to currentColor. Given that it stays as a keyword at computed value time, one could think that it transition / animations between currentColor and other colors are meant to be discrete, but all browsers actually interpolate. This should be made explicit in the spec.

This could probably be done by adding the following sentence to the paragraph.

Interpolating to or from currentcolor is possible. The numerical value used for this purpose is the used value.

[upsuper]

I think this is just a dup of #445.

[frivoal]

That's right. Thanks.