It appears you are using an HTML 5 theme. The W3C doesn’t validate all HTML output even though it might be “valid code”. The errors you’ve sent are generated by WordPress and most can be removed fairly easy by de-registering the WordPress Hooks which are causing them. The hooks are used to achieve things like grouping galleries, categories, and other similar tasks. Some can be removed without a negative or noticeable result. If you’re trying to be 100% W3C Valid, it might be best to stick with XHTML until HTML5 becomes more accepted/documented. I believe the time you’ll spend to remove the errors out way the benefits of using HTML 5. I would either live with the errors or wait for the W3C to expand the rules. If you are set on using HTML5 and achieving 100% Validity, here are 2 informative links to help you out: WP-Engineer Article on “Cleaning up the WordPress Header” Articles describing the cause of the Keyword Validation Errors.
