I have run into this a couple times in visual diff testing and the latest instance is this diff on enwikitionary. While there are other reasons that this rule doesn't apply to Parsoid HTML, the ".mw-parser-output > h2" bit of the selector will fail on Parsoid output because of intervening <section> tags.
What is the best way to handle this?
If we plan to strip section wrappers for read views, this will get resolved naturally. Otherwise, we'll need to communicate and/or tweak CSS rules on wikis ourselves. This can be a bit cumbersome.