General CSS EcmaScript Cross-Browser DHTML
W3C the World Wide Web Consortium
W3 Validator validate your web pages.
W3C Mailing List
HTML 4 in Netscape and Explorer ... a great resource documenting differences in support. More wisdom from Webreference.com.
Yuriko Ienaga lists every single HTML 4 tag, and shows which browsers support it (or don't) at www.ncdesign.org/html/list.htm. An outstanding effort.
HTML 4.0 as handled by Navigator and Explorer. An earlier piece on the same subject.
Stuck With Old Browsers Until 2003? Jakob Nielsen fears that we will be.
Amazing HTML covers most areas of webdesign from CSS to HTML to basic CGI, FTP, and more; geared toward beginning to intermediate web authors
Authoring Valid HTML builder.com reviews various validators
Best Viewed with Any Browser campaign to eliminate Netscape (or Microsoft) dependance from web design; worth seeing
Bobby accessibility/browser compatibility checker
HTML Compendium originally a simple listing of HTML elements, it has expanded to include CSS and an extensive "Notes" section.
HTML Reference Library
HTML Standards Compliance: Why Bother? from the Web Developer's Virtual Library
A List Apart high signal, low noise mailing list and weekly magazine for people who make web sites.
HTML Writers Guild
Index Dot HTML
Project Cool Developer Zone tutorials and advanced topics for developers
Useit.com Jakob Nielsen on developing usable sites. Nielsen is a web usability consultant and former Sun Microsystems Distinguished Engineer.
The Web Developer's Virtual Library
The Webmaster's Reference Library
WebMonkey Hotwired's help and tutorials