Digital Web:

"CSS can be ideal for use by information architects and interaction designers for rapidly developing prototypes for usability testing, review, and analysis. [...] For those who want to get more advanced with rapid prototype development, data-driven pages and database queries can be simulated by storing data in XML and using XSL/XSLT to apply designs. Since XML can be processed automatically in some browsers, it is possible to create a prototype simulating a database interaction without database or server access."

This all sounds very attractive, but having CSS to properly work is such a PITA that people might waste precious time debugging prototypes, which seems like the last thing you want to do. Give me more wizard-based WYSIWYG "I don't need to look at the code" support of CSS and XML data islands in authoring tools, and yes, such smart prototypes that actually do something would kick ass.