Flying Saucer is a project started by Joshua Marinacci, hosted on java.net website. The idea is to have a standards-compliant renderer for XHTML (and XML), using CSS 2.1 for styling. I got involved in a re-write of the CSS property handling in the summer of 2004.
We just released version R5 (0.5) of the codebase--a big improvement over version R4 from October of last year. I worked on improved CSS parsing, lookup and resolution, a resource (XML, images) loading API, XML catalog support, a testing application (Eeze), logging and configuration. You can run the sample "browser" application using Java Webstart by clicking here. Visit the project website to download the code.
The project is making good progress. Josh had a good foundation to start with--one reason I joined was that even after a 0.2 release, you could already see basic HTML pages styled with CSS. This is the first FOSS project I signed up to where I have actually contributed code that shipped. It's been pretty cool. Josh is in Atlanta, /tobe is in Sweden, and I in Berlin. We do a work-shift sort of thing.
If you are a Java programmer, it is worth checking out.