Every selector of a comma list cloned the whole declaration block, and every
element scanned every rule, so a sub-megabyte <style> could demand billions of
string pairs. Selectors now index one shared block, the sheets are capped at
20 000 selectors and 50 000 declarations, rules are bucketed by id/class/type
so an element checks only candidates, and matching work over all elements is
charged against a 50M-step budget that fails the import instead of doing it.
Barzabel
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