How can I check the HTML, XHTML, CSS and other web standards used on a web page?

For help validating web pages which use HTML, XHTML, CSS, or other web standards, visit the World Wide Web Consortium web site, which provides a set of online validation services. For more information, see the Validators and Lints section of the Google directory.

Also see:

• What is HTML, and where can I find a guide for creating HTML documents?
• What is CSS?