The W3C this week recommended Extensible Stylesheet Language (XSL). The spec dictates how XML documents can be displayed and formatted, and is, in a basic sense, the XML-version of Cascading Style Sheets (CSS).
CSS is still valid to use for X/HTML-based documents, or, as long as no structural changes are required, XML-based ones.
Used with XSL's sister recommendation XSL Transformations (XSLT) — which was approved in November of 1999 — Web developers can manipulate XML data and render it to a readable, formatted page.
For example, XSL and XSLT can use the same XML-source to create a Web page or a book. XSL allows for complex layouts thanks to its reliance on unique formatting objects (for example, "footnote", "subhead") and properties.
The W3C's XSL site has more information, including tutorials and mailing lists.