[css-scrollbars] Inheritance, initial values

[ewilligers]

Properties inherit or not according to the spec.
Properties have initial values according to the spec.
https://drafts.csswg.org/css-scrollbars/#property-index

[ewilligers]

The inheritance tests were failing in Firefox when
scrollbar-color was light and scrollbar-width was 10px.

Changed the values used for inheritance testing, to allow the
inheritance tests to pass with the current Firefox implementation.

(The scrollbar-color initial value test still fails in Firefox.)

[upsuper]

It probably shouldn't test scrollbar-color initial value, because I consider it be following "A resolved value special case property like color defined in another specification" so the resolved value should be used value.

It's not completely clear whether it's what we want, though... Actually having auto resolved to auto is easier to implement than the current approach, to be honest.

[upsuper]

Filed w3c/csswg-drafts#3237 for that.

[upsuper]

The latest nightly should pass all tests here now.

[ewilligers]

The spec is clear that auto is the expected computed value for scrollbar-color initial value:

Initial: auto

Computed value: keyword or two computed colors

If this property computes to value other than auto

@tantek Please review.

[upsuper]

But the result of gCS is resolved value, which is not necessarily computed value. Anyway the group has resolved on have keywords keep being keywords in resolved value.

[ewilligers]

But the result of gCS is resolved value, which is not necessarily computed value.

Thanks. I made the same mistake with caret-color and outline-color tests. Fixing those in
PR 13748

[ewilligers]

@upsuper Please review. I have updated the commit message so it no longer mentions Firefox.

[upsuper]

LGTM. auto should work now given the resolution a while back.