The CSS2.1 Test Suite still needs a lot of work, and the CSS Working Group can't finish it all by itself. Anyone can contribute: here are the guidelines.
The test suite is licensed under both the W3C Test Suite License and the 3-clause BSD License.
To give us permission to distribute your contribution under these two licenses, you need to fill out W3C's license grant form (unless you or your company is a member of the CSS Working Group, in which case you're covered already).
You can submit a new test by
Please make sure submissions follow the CSS2.1 Test Suite Guidelines. New tests are subject to the review process. We may ask you to fix problems in your test and resubmit it before adding it to the test suite.
To contribute a fix to an existing test, send an email to public-css-testsuite@w3.org with both
Fixes to existing tests are also subject to the review process.
If you've waited at least a week and your contribution seems to be going nowhere, please complain to the test suite owner (currently fantasai). No, really, please do complain if this is the case; I'm probably just having some trouble staying on top of things and need a reminder or three.