The World Wide Web Consortium has been in existence since 1994, and its purpose is to set standards for the web. For any standards-related question, this is the place to go. This is also the place to get your HTML and CSS validated.
W3C.org
On-line tutorials with try-it-yourself examples and quiz tests. Topics include: HTML, XHTML, CSS, XML, XSL, DTD, DOM and WAP.
W3Schools.com
Portal leading to a large number of excellent resources
Internet.com
One of my favourite web development forums, covering all aspects of web development.
Builder Buzz
Another favourite forum, with a very friendly atmosphere. Some of the forums are members-only.
Cyber Crew
On this page Transitional HTML 4.01 and CSS 1 are used. If you're seeing this text you either have CSS switched off in your browser, or you're using a browser that can't handle CSS. If you're using an older browser version, you might want to consider upgrading.