The only reason why we had the `existing_style_for_style_damage` bit is to apply
some optimizations that we don't have anymore.
I still want to reintroduce a few of them, at least for the non-eager
pseudo-element case... But I think I won't need this at all.
This allows us to remove a fair amount of Gecko code too.
Stylo: match both ::placehoder & ::moz-placeholder for placeholder ps…
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The first patch of "Bug 1348490 - stylo: need support for ::-moz-placeholder pseudo element"
Part 2 need to be landed immediately after Part 1 be merged into servo repo:
https://reviewboard.mozilla.org/r/141264/diff/9#index_header
Bugzilla link:
https://bugzilla.mozilla.org/show_bug.cgi?id=1348490
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