Last modified: 2013-03-19 00:06:32 UTC
Issues detailed in http://www.mediawiki.org/wiki/Parsoid/Todo:HTML5
Jan Christiansen looked a bit into this and sent me some info by mail: The CSS spec allows comments anywhere, so this is probably a bug in the HTML5 treebuilder (unlikely) or JSDOM. I strongly suspect JSDOM because I think I have seen a case where this also happened to DOM nodes we manually created.
Closing, as we switched from JSDOM to Domino which does not normalize CSS rules any more.