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The new behavior is behind the 'CustomStatePseudoClass' runtime flag.

Explainer: https://github.com/w3c/webcomponents/blob/gh-pages/proposals/custom-states-and-state-pseudo-class.md

Bug: 1012098
Change-Id: Ia0d1e145cf6589868c7c39d3b1c51fdf2b41bc68
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1800138
Commit-Queue: Kent Tamura <tkent@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#704556}