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At the time of writing this tutorial HTML5 is very much in initial stage and there are only few validtors available on the net.
But following two validators are very authentic and can be used:
The W3C Markup Validator checks the markup validity of Web documents in HTML, XHTML, SMIL, MathML, etc. This validator is part of Unicorn, W3C's unified validator service.
To use this validator for HTML5, you need to use More Options and select Document Type as HTML5 (experimental) as shown below.
Here is the another currently known HTML5 validators: Henri's Validator.nu (X)HTML5 Validator (Highly Experimental):
This validator has compatibility problem, so you can try it in lower versions of IE or Mozilla.