[a] (3) Suggested alternative text for anciliary images.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26232
Affected topics: HTML

git-svn-id: https://svn.whatwg.org/webapps@8789 340c8d12-0b0e-0410-8428-c7bf67bfef74