Currently we set important_rules_change to true in compute_style()  whenever we process MatchAndCascade. If we refine it, we can avoid needless SequentialTasks. I noticed this fact when I run test_restyles.html on stylo.  https://hg.mozilla.org/mozilla-central/file/1d042bcb2632/servo/components/style/traversal.rs#l669
status-firefox57=wontfix unless someone thinks this bug should block 57
status-firefox57: affected → wontfix
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