A valid website is a website that uses valid HTML or XHTML code according to the W3C, or World Wide Web Consortium. The W3C promotes web standards, universal sets of programming guidelines designed to improve web usability and minimise badly constructed code. Luckily, the W3C runs a validation service that allows you to instantly check to see if any website is valid.
Open your web browser and navigate to Validator.W3.org.
Input any website URL in the box next to "Address," then click "Check."
Examine any errors and warnings the validator finds--the vast majority of websites have at least a few validation errors. If you own the website in question, the errors will usually include tips on how to correct the problems.