Here are some of the relevant web sites however any web site on the topic may well be of use.

Web Design

• The Yale Style Guide probably the best single source of information about web design
• Web Design Group - Cascading Style Sheets
• Web standards software and learning
• W3 Schools - covers CSS and XML
• Zen Garden for some interesting interpretations of CSS
• Web reference articles on CSS
• Eric Meyer on CSS
• and of course A List Apart

Extensible Markup Language (XML)

• XML .com has excellent information on XML
• Webreference has articles on XML
• Webmonkey has some interesting articles on a range of topics including XML
• XML Basics - an introduction to XML from XML Files.com

Extensible Stylesheet Language (XSL)

• W3 Schools has a thorough tutorial on Extensible Stylesheet Language for Transformations (XSLT)
• XML Files.com have a tutorial called "XSL - An Introduction to XML Extensible Stylesheet Language"
• Webmonkey has some interesting articles on XSL