Apparently no one has informed
http://big.faceless.org/ of this, their Report Generator product uses XHTML + CSS2 to produce professional-quality PDF output, simpler and faster than FOP, I might add. ;-)
CSS is not a transformation language like XSL or DSSSL, so I don't quite get that comparison, unless you're referring to XSL-FO, which is contained in the XSL spec, I suppose. According to
http://www.w3c.org/Style/CSS/ CSS is "a simple mechanism for adding style (e.g. fonts, colors, spacing) to Web documents," although I'd add that while it was originally intended for continuous media, it has extensions for Paged Media that people use professionally. I see XSL and XSL-FO as complementary technologies to CSS, facilitating an easier way to describe text flow in cases where CSS even with Paged Media extensions won't quite do. But in CSS2.1 with Paged Media extensions, one can do describe quite a lot of styling without ever having to resort to FO, although XSL is still required for things like Table of Contents generation and indexing.
What interests me about Prince 4.0 as described was the ability to go directly from Docbook+CSS to PDF, rather than a 2-step (using XSL) to XHTML+CSS and then PDF. I'm also open to any other suggestions.