Essential tools for maintaining website code compliance, the W3C or World Wide Web Consortium provides a several quality assurance validators to check the code validity of Web documents in HTML, CSS, WAP, XHTML, SMIL, MathML, etc. Your first stop on the list should be The Markup Validation Service, as it will check the markup of the web page. Simply enter the URI of a document you would like validated in the textbox provided. The informative error messages are helpful in finding and fixing any errors. Once your markup is valid, it is time to move on to your CSS. First format and optimize your CSS for FREE using CLEANCSS. Then return to the W3c tools for validation.
To validate your CSS the W3C provides the CSS Validation Service for Check Cascading Style Sheets (CSS) and (X)HTML documents with style sheets. Simply enter the URI of a document you would like validated in the textbox provided. There is also the W3C mobileOK Checker for those seeking to publish WAP content. A full list of validators is available as well as QA Matrix comparing the capabilities of each. The W3C validators are hosted on server technology donated by HP, and supported by community donations.