W3C HTML 5 Conformance Checker
Karl Dubost: We are happy to announce that W3C has integrated a version of HTML 5 conformance checker into a beta instance of the W3C Markup validator. That will help us to detect bugs, improve the user interface, and benefit from the large W3C communities.
Gotta love the “random” page that Olivier Théreaux chose to mention in his announcement.