[css-ui][css-cascade] revert-layer and appearance

[frivoal]

In #7224 (comment), @Loirooriol said:

What about

@layer {
  .widget { border-left-style: solid }
}
@layer {
  .widget { border-left-style: revert-layer }
}

The cascaded value is revert-layer, but the native appearance should be disabled anyways since the specified value will be border-left-style: solid.

That's a very good point. From an implementation point of view, is it practical to examine the cascade, and pop cascaded revert-layer until you get to a cascade value that's neither revert nor revert-layer, so that you can see if that value is an author stylesheet or in the ua stylesheet?

[Loirooriol]

Already filed #7239