Node::HasIDFlag and Node::HasClassFlag can be replaced with ElementAttributeData, which doesn't need any hashtable lookup.
Created attachment 126208 [details] Patch
Ryosuke, Andreas, could you take a look? Note that I don't see any significant performance impact on Dromaeo.
Comment on attachment 126208 [details] Patch Great idea! r=me
Waiting Bug 78070 is resolved as rniwa's suggestion at irc.
Committed r108449: <http://trac.webkit.org/changeset/108449>