Notice most of these sites refer solely to HTML code. Remember to make adjustments so you're coding in XHTML!
Instructions and Help
W3Schools.com. THE site for web tutorials, based on W3C Web standards. HTML AND XHTML tutorials available, among lots of others. Nominated for World's Best Web Site.
WebHelp. A sort of
a cross between a search engine and a help desk, this site has real people on-line
to help you with real questions.
Cedge's HTML Cheat Sheet. This is a very good resource for all kinds of HTML code (notice that this is HTML not XHTML--code properly). Check out the "Mythical" Vertical rule code.
Trouble-shooting
W3Schools.com. This site has HTML and XHTML code validators. You're free to use these on your Web projects.
Ask Dr. Website. This site contains an archive of questions and answers on HTML problems. There's also an e-mail link to ask a new question, if your problem hasn't been addressed before.
Dr. Watson. The HTML equivalent of a spell and grammar check, and it's free, unlike some other sites of this type. You are free to use this site's coding checker on your projects.
Design Do's and Don't's
Web Page Design Flaws. This site was recommended by Kristen Joel and Jon Laughlin. It's a page of links and other resources that addresses many issues of web page design. Check it out!
Web Content Accessability Guidelines. Guidelines intended for all web content developers to promote Web accessability to people with disabilities (Section 508 Compliance).