I've looked at ruleset invalidations for Facebook. The problem with scheduling all invalidation sets on the root element means we'll often have rules which are universal in the rightmost compound. These selectors show up in my printf for navigating my Facebook profile:

ForceSubtree: ._5v3q ._58jw :first-child
ForceSubtree: ._5v3q ._58jw :last-child
ForceSubtree: ._53r8 > :nth-child(1n)
ForceSubtree: ._2m3o :first-child
ForceSubtree: ._2m3p._5a5j :first-child
ForceSubtree: ._2m3p._5a5l :first-child
ForceSubtree: ._2m3p._5a5k :first-child

That means for Facebook, and I suspect many (most?) other pages, would end up with always doing full document recalcs.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6e397619af9370372f49a4d1281d1f19b013f501

commit 6e397619af9370372f49a4d1281d1f19b013f501
Author: rune <rune@opera.com>
Date: Mon Jan 09 20:42:15 2017

Added trace for scheduleInvalidationsForRuleSets.

R=esprehn@chromium.org
BUG=678849

Review-Url: https://codereview.chromium.org/2620673002
Cr-Commit-Position: refs/heads/master@{#442332}

[modify] https://crrev.com/6e397619af9370372f49a4d1281d1f19b013f501/third_party/WebKit/Source/core/dom/StyleEngine.cpp

Interesting, I guess lets see how expensive that tree walk is on typical content. Solutions would be:

- Simple: Schedule subtree invalidation sets for the left most selector.
- Hard: Teach the invalidator about two pass selectors. So we first match the left most component, and upon matching it, schedule an invalidation set for the right most component.

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