How to :HTML5 Validation

The W3C Markup Validator:

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 selectDocument Type as HTML5 (experimental) as shown below.

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(2) The Validator.nu (X)HTML5 Validator:

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.