[e] (0) introduce stable IDs for 'embedded content' sections
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=24694
Affected topics: HTML, Rendering

git-svn-id: http://svn.whatwg.org/webapps@8506 340c8d12-0b0e-0410-8428-c7bf67bfef74