The CSS2.1 Conformance Test Suite tests conformance to the CSS2.1 specification. It is currently in the earlier stages of development, and is very incomplete. Additionally, some tests may be invalid due to changes in the spec. The latest version is available on the W3C website. The source files are available on the repository website. Anonymous read-only Subversion access is available to this repository. Note that many of the tests rely on the Ahem font.
Discussion about the CSS2.1 Test Suite takes place on the publicly-archived public-css-testsuite@w3.org mailing list.