W3C is the World Wide Web Consortium and since 1994, the W3C has provided the guidelines by which, websites and webpages should be structured and created. Here are the links to validate your web pages:
* Validate HTML/XHTML File against W3C Standard HTML/XHTML Validator.
* Validate CSS File against W3C Standard CSS Validator.
While verification, you may get errors along with appropriate reasons. All the validations will be done using XHTML DTD, which is a refined version of HTML.