[ac] (0) Allow <h1> (and company) inside <summary>, so that you can give a <details> element a heading.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24372
Affected topics: HTML

git-svn-id: http://svn.whatwg.org/webapps@8423 340c8d12-0b0e-0410-8428-c7bf67bfef74