approved/ is for tests that have been reviewed and approved. Approved tests are managed as a common resource. Do not touch anything here unless you are a test suite Owner or Peer, or have permission from one and know what you are doing.
contributors/ contains a directory for each test suite contributor. Tests inside a contributor's directory are managed by that contributor. Do not touch anything outside your own directory unless you have permission from the directory's owner or from a test suite Owner or Peer.
tools/ is random scripts that may be useful when administering tests.
Absolutely everyone is welcome (and even encouraged) to contribute to test development, so long as you fulfill the contribution requirements detailed in the Contributing Guidelines. No test is too small or too simple, especially if it corresponds to something for which you've noted an interoperability bug in a browser.
To gain write access to this repository, sign up for an account on the CSS Test Suite Manager (aka Shepherd) at: https://test.csswg.org/shepherd/register and then submit a request on the Repository Access page at: https://test.csswg.org/shepherd/account/access/
You will be notified by email when your request is processed.