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http://html-differences.whatwg.org/#new-elements [[ hgroup represents the header of a section. ]] obsolete in 5/5.1 http://www.w3.org/html/wg/drafts/html/master/obsolete.html#non-conforming-features
https://github.com/whatwg/html-differences/commit/3cb56b5ff78c4cef4a74e3eef18b06822c01d1b4