In the initial implementation of scrollbar-color in Gecko, I followed the way color properties are handled for gCS, basically according to the following sentence in CSSOM spec:

A resolved value special case property like color defined in another specification ... The resolved value is the used value.

But it is not clear to me whether this is actually useful at all, and should we make it follow other color properties to be the "special case" for gCS.

Although Gecko currently implements the "special case" way, i.e. auto is computed to two colors, I'm happy to make it not the case, because that's actually simpler. Also with the introduction of light and dark, maybe it makes sense to say auto doesn't necessarily mean two colors, so it shouldn't be resolved to two colors?

Anyway, I think this needs to be made clear in the spec one way or the other.

cc @tantek