Some nice tutorials for learning HTML:
http://www.webpageworkshop.co.uk/?table=others&file=html_index.xml&menu=html
A reference chart to the most commonly used HTML elements and tags:
http://www.webpageworkshop.co.uk/?table=others&file=html_reference.xml&menu=main
A wealth of good HTML information and advice (including an online HTML validator):
http://www.htmlhelp.com/
For learning and using HTML appropriately, and doubling up as a good reference:
HTML & XHTML: The Definitive Guide, 5th Edition By Chuck Musciano, Bill Kennedy ISBN: 0-596-00382-X, Pub: O'Reilly, £28.50 UK
For a good reference only book:
HTML Pocket Reference, 2nd Edition By Jennifer Niederst ISBN: 0-596-00296-3, Pub: O'Reilly, £ 8.95 UK
Alertbox: Jakob Nielsen's Alertbox. An archive of articles on Website Useability. Indispensable for the professional Webmaster:
http://www.useit.com/alertbox/
Getting Started: Making a Web Site Accessible; from the World Wide Web standards authority:
http://www.w3.org/WAI/gettingstarted/
Information Design for the World Wide Web:
http://www.washington.edu/webguides/design.class/index.html
Web Style Guide. Formerly known as the Yale Style Guide. A complete style guide book online:
http://www.webstyleguide.com/
Multimedia Design for the World Wide Web. The basics of good Web design:
http://archive.ncsa.uiuc.edu/General/Training/AdvHTML/multimedia.design.html
A straight forword - converting how to:
http://www.webpageworkshop.co.uk/?table=others&file=xhtml_index.xml&menu=xhtml
Another good conversion guide:
http://www22.brinkster.com/beeandnee/techzone/articles/htmltoxhtml.asp
More information about the differences:
http://www.vbxml.com/xhtml/articles/html_to_xhtml/default.asp
For learning and using XHTML appropriately, converting from HTML, and doubling up as a good reference:
HTML & XHTML: The Definitive Guide, 5th Edition By Chuck Musciano, Bill Kennedy ISBN: 0-596-00382-X, Pub: O'Reilly, £28.50 UK
An article introducing and explaining XML:
http://hotwired.lycos.com/webmonkey/98/41/index1a.html
An XML portal:
http://www.xml.com/
The Semantic Web - the future of the World Wide Web:
http://www.sciam.com/article.cfm?articleID=00048144-10D2-1C70-84A9809EC588EF21
A complete online tutorial for learning CSS:
http://www.w3schools.com/css/default.asp
CSS reference:
http://www.w3schools.com/css/css_reference.asp
Online testing for your CSSs:
http://jigsaw.w3.org/css-validator/
Another online CSS testing service but with more stright forward error reporting. Very good but only handles CSS as a stand-alone file (i.e. not styles within a HTML page):
http://www.htmlhelp.com/tools/csscheck/