Website Code Validator
Using the validator is easy. Just enter the URL of your website, or any website, and use the check button to see if there are any erros in the html markup.
External resources and incorrect use of html elements are just a few of the problems that can cause your site to fail validation, but there could be other problems that the validator cannot detect. Valid Html and Css don't mean everything is okay if It's all green. It shows there's no syntax erros in you markup and that's a plus, but that don't mean you website is fully standard compliant.
Your codes could pass Html/Css validation and fail search engines, usability, and accessibility standards, which might be the most important of all the web standards requirements. Almost every aspect of designing and building websites requires validation for adherence to existing standards established by The (W3C) World Wide Web Consortium, the international standards organization for the World Wide Web.