Our conformance test suites: how to run them, known bugs, and how to contribute. The tests are maintained in the W3C's dev.w3.org cvs repository.
Test suite discussion takes place on the public-css-testsuite mailing list. The #css-test channel on irc.w3.org:6665 is also available for discussion, and we have weekly IRC “Test Days” there on Wednesdays.