This allows us to traverse NAC as well. See bug 1292618 comment 3.

We'll need to make sure we don't double-process any NAC-backed pseudos (::before and ::after, and the HTML5 form control pseudos - pretty much everything except ::first-letter and ::first-line). I wonder how that should work...

I guess during parallel traversal we'll want to expose NAC to Servo, but _only_ NAC that doesn't correspond to a PE (since those are handled by the hashmap). And we'll want to avoid traversing them in the case that the change hint is ReconstructFrame, since that'd be wasted work. Then, during SC fixup, we can just use AllChildrenIterator. For PE NAC, we use the style from the hashmap. For Non-PE NAC, we use the style that parallel traversal cached on the NodeData.

This was fixed with incremental restyle.