Instead of applying animations and transitions to styled elements,
include them in the cascade. This allows them to interact properly with
things like font-size and !important rules.
This begins to address #26625 by properly applying CSS variables during
keyframe computation and no longer using `apply_declarations`. Instead,
walk the declarations, combining them into IntermediateComputedKeyframe,
maintaining declarations that modify CSS custom properties. Then compute
a set of AnimationValues for each keyframe and use those to produce
interpolated animation values.
Instead of recalculating the animation style every tick of an animation,
cache the computed values when animations change. In addition to being
more efficient, this will allow us to return animation rules as property
declarations because we don't need to consult the final style to produce
them.
There is no need to set the dirty descendants flag unsafely from the layout side
for elements with pending restyles, we can do that on the DOM side when draining
the restyles from the Document.
For clarity, I introduce <LayoutDom<Element>>::parent_node_ref to contain
the remaining unsafety bits out of composed_parent_node_ref which is more
complex than just a field access.
They don't do anything fancy so there is no additional unsafety calling them
compared to using LayoutDom in the first place, the usual story of all
those changes.
This safe method is the basic block to access element attributes from layout.
We reuse it in the other attr-related layout methods to remove a pretty big
source of rampant unsafe code between script and layout.
