http://validator.w3.org/

http://valsvc.webtechs.com/

http://www.cast.org/bobby/
(Also checks for a page’s accessibility to people with disabilities.)

http://www.ozemail.com.au/~dkgsoft/html/

Appendix: HTML Reference Guide

At the time of writing, the W3C had just finalized the HTML 4.0 specification, and the Netscape nor Microsoft browsers supported the HTML 3.2 specification. (Most likely, they will both release new versions when the CSS definition is finalized.) If you wish to be write HTML that conforms strictly to the standard for a particular version of HTML refer to

Alternatively, if you do not care about the standard, it is suggested that before you invest heavily in the use of an HTML tag, you check its functionality, first with the Web browser(s) you use, and then with the Web browsers you anticipate being used to access your information server.

This is not a summary of all the features of HTML 4.0 or 3.2; Chapter Nine covers where to go for that information. Rather, it is intended as a quick reference to the major features of HTML. If you need more details in a hurry refer to the Yahoo page on HTML--

--or see the W3C’s HTML 4.0 specification page: