[giow] (1) Fix IDL blocks due to my misunderstanding what 'optional' meant
Fixing https://www.w3.org/Bugs/Public/show_bug.cgi?id=26562
Affected topics: Canvas

git-svn-id: https://svn.whatwg.org/webapps@8783 340c8d12-0b0e-0410-8428-c7bf67bfef74