Re: [fullscreen] Styles of former fullscreen elements (#18)

Since we have a single `Document.fullscreenElement`, I think `:fullscreen` should keep its current definition, matching at most one element per document, but we can make whatever other changes are necessary to top layer to make the style be what it needs to be.

I can't make a very concrete proposal, because the problem still isn't very clear to me, other than that the computed style can now be different from before the changes, and this can be observable. Some examples of where it'll result in unexpected results might help.

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