[giow] (3) Adjust the rules on rendering images without 'alt' attributes.
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=23443
Affected topics: Rendering

git-svn-id: http://svn.whatwg.org/webapps@8281 340c8d12-0b0e-0410-8428-c7bf67bfef74