Web Page Validator
A validator is a software program that can check your web pages against the web standards.
When using a validator to check HTML, XHTML or CSS documents, the validator returns a list of errors found, according to your chosen standard.
Make sure you make it a habit to validate all your web pages before publishing.
Validate your HTML files with W3C
Validate your CSS files with W3C
Validate your XHTML files with W3C
An XHTML document is validated against a Document Type Definition (DTD).
You can read more about XHTML validation in our HTML tutorial.