Years ago, Dan Connolly created one of the first HTML validators. Since then, validation is one of the practical techniques to maintain quality of your documents. It is not the only one but one which has a lot of influences on developing a language.
HTML 5 is a specification in development. Many implementers have started to integrate the technology into their own softwares: browsers, parsers, etc. Henri Sivonen has developed a parser in Java for his conformance checking service.
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.