Anyone can contribute: here are the guidelines.
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).
First, send us permission to use the test as described above.
You can either submit tests into the test repository yourself (see below), or you can send it as a link or attachment to firstname.lastname@example.org and ask someone else to submit it. If you are contributing tests for CSS2.1, you can also contact Gérard Talbot for assistance.
To obtain write access to the repository, log in to Shepherd and submit a request to modify assets on the Repository Access page. You will receive an email letting you know when access has been granted. We will set up a submission folder for you in the test repository. If you don't already have an account on Shepherd, you can register directly in Shepherd. Note that this wiki and Shepherd use the same accounts, so if you have an account here, you already have an account on Shepherd too.
To submit a test, simply add it to the
submitted folder in your contributor directory and push your changes to
https://hg.csswg.org/test. It will show up in Shepherd, our test-tracking system. You can use the
incoming folder in your contributor directory as scratch space. See How to use Mercurial.
Please make sure submissions follow the CSS2.1 Test Suite Guidelines and are in the proper format. 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 email@example.com with both
Fixes to existing tests are also subject to the review process.