Removing recursion from ComplexSelector
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Using a smallvec instead of recursively calling to_css for ComplexSelector to handle running out of stack.
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Also, simplify all the pre-snapshot attribute hacks in the script and style
code.
MozReview-Commit-ID: 6c9ipeb7Tnr
Signed-off-by: Emilio Cobos Álvarez <emilio@crisal.io>