From Homestar Runner Wiki
The validation committee helps to ensure that all wiki pages validate as proper XHTML and CSS. This means stripping out bad code, replacing it with good code, and helping to educate our community on what tags can and can't be used in valid code.
 First order of business — font tags
There are many <font color="red">text</font> tags hanging out here and there. Many people are using them in their sigs. Since font tags are allowed in XHTML 1.0 Transitional, it's okay if people want to continue using them, but they should make sure to enclose the color declaration in quotation marks to keep things valid. However, we should try to switch everyone to <span style="color:#rgb;">text</span>. This method is much more forward compatible.
 Second order of business — tag ordering
Another major problem that people tend to do is end their tags in the wrong order. For example, someone might put ''[[Homestar Runner'']]. This causes major issues with XHTML (or any HTML for that matter) and should be rectified to ''[[Homestar Runner]]''.
 Firefox validation extension
The HTML Validator is available for Mozilla Firefox users. It uses a built-in parser to check the validity of a page as after it is rendered; the results of which are displayed in the status bar. This extension is recommended for all validation committee members who use Firefox.
 Internet Explorer Developer Toolbar
Similar functionality can be enabled in Internet Explorer versions 6 and 7 by using the Developer Toolbar. This toolbar allows a user to open the W3C validation for that page. Users of Internet Explorer versions 8 and 9 now may access this functionality by pressing F12. An user can then open the "Validate" menu located at the top of the window, and choose "HTML".
 Opera validation
 Online Validator
Pages may be validated online using the.