Most of the sites are being standardized with W3C Web Standards. W3C provides lot of web standard services to the web. They have the web specification, process and documentation for all the web standards. You can apply HTML, CSS and Accessibility standards to your web sites well. W3C provides the better validation services to all. It is all just to follow the standards to make it better in online. These web standards are globally accepted by the people who are in the web.
You can validate your site HTML, XHTML,CSS and Accessibility design structure.
HTML, XHTML Validator:
You have to follow the standards strictly to have W3C standards for your web site. After validation you can have the W3C logo approval for your site.
All the best!