CSS
Books, Browser Compatibility, Commercial Services, Examples, FAQs, Help, and Tutorials, Reference, Tools, Using with XML, About.com - Cascading Style Sheets, Code Style - CSS Style Guide, css-discuss Mailing List, Dive into Mark: CSS, Eric Meyer - CSS, EServer TC Library: Cascading Style Sheets, RichInStyle.com, Style Sheets by Example, The CSS Pointers Group, W3C - Cascading Style Sheets
CSS and XSL: which should I use?
Brief article from W3C with a diagram of the markup-to-presentation process for HTML and XML documents.