Web Design: Proper HTML, XHTML, etc

Proper usage of HTML and XHTML is very important for websites. Although your website may render properly on your browser with incorrect markup, different browsers treat different errors in HTML differently and you site may look horrendous to visitors on other browsers.

Below is a tool to validate your markup code.

W3C Markup Validator is a service provided by the W3C, World Wide Web Consortium, to help webmasters check the validity of their code. These folks are the people who set the standards for HTML, XHTML, and other markup languages.

In addition to proper usage of HTML, there are other guidelines for web accessability, site optimization and other facets of web design. Unfortunately there is no tool to check if you meet Web Content Accessability Guidelines, but the other guidelines have tools with which to check.