The W3C have just launched a new markup validation service that, unlike their previous HTML validator, can validate your new shiny HTML5 code. The W3C or World Wide Web Consortium is the international standards organization for the World Wide Web. They are they splendid folks who create the specifications for things like HTML and CSS.
The W3C has launched the Nu Markup Validation Service, a non-DTD-based markup validator being made available to the community in parallel to the existing DTD-based validator.w3.org W3C markup validator. The W3C Nu Markup Validation Service uses the same backend as the Validator.nu site, which is also the backend for the HTML5-checking feature of validator.w3.org. The Nu Markup Validation Service is a separate, standalone validator which provides that same HTML5-checking feature while also offering a user interface that exposes additional options, such as full validation support for XHTML5 documents, and the ability to validate documents that contain features from RDFa Core 1.1 and from RDFa Lite 1.1.
You can check out the new validator service here http://validator.w3.org/nu/ and find out more about the validation engine here http://about.validator.nu/