Gather CSS mutation operations into trigger(s)

[mdittmer]

We need to approximate CSS change detection. This is probably not worth doing perfectly; things like :hover and declarative animations may require observer patterns that are too aggressive (or simply undetectable).

That said, we should look out for adding/removing style sheets, setting properties on CSS style objects (in Javascript), etc..

Later, this could be combined with a CSS parser to try to fill in the gaps created by more subtle, unobserved, or unobservable CSS mutations.