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The rev attribute is the first obsolete attribute listed in the "Obsolete Attributes" section of the "Differences from HTML4" document: http://www.w3.org/TR/html5-diff/#obsolete-attributes It should be listed in the "Non-conforming features" section of the spec as well - http://www.w3.org/html/wg/drafts/html/master/obsolete.html#non-conforming-features
I submitted this bug but it's not important enough to me to keep open.