Attribute selectors are increasingly common and we have them in our default style sheet too. [foo] and [foo="bar"] type selectors can be resolved quicky and easily in the fast path.
Created attachment 109318 [details]
This is ~20% progression in styleForElement() performance loading the full HTML5 spec (~0.8s).
Attachment 109318 [details] did not pass style-queue:
Failed to run "['Tools/Scripts/check-webkit-style', '--diff-files', u'Source/WebCore/ChangeLog', u'Source/WebCor..." exit_code: 1
Source/WebCore/css/SelectorChecker.h:87: The parameter name "attribute" adds no information, so it should be removed. [readability/parameter_name] 
Total errors found: 1 in 7 files
If any of these errors are false positives, please file a bug against check-webkit-style.
Created attachment 109320 [details]
now with more style, less printfs