Validates the HTML string/file for well-formedness and compliance with w3c standards. It tries to make use of the doctype declaration to evaluate the document structrue but will resort to best practice if unmatched. The validator will report on missing or invalid attributes, unknown tags, unclosed tags and more.
*This validator does NOT support HTML 5! I leave it here for backward validation, but if you need a HTML 5 validator, try the following site HTML 5 - Validator.Nu.