Good test suites drive interoperability. They’re a key part of making sure Web standards are implemented correctly and consistently. The W3C hosts the official CSS Conformance Test Suites here. Many of these test suites are still works in progress.
To keep up with the CSS Working Group's latest announcements, discussion topics, and resolutions you can follow the CSSWG weblog, which is aggregated with other CSS-focused streams on The Future of Style.
Finally, detailed technical discussion about the CSS specifications takes place on the publicly-archived www-style mailing list.
Want to get involved in shaping the future of CSS? Learn how to read the CSS specs and follow the discussion on The Future of Style and www-style. You can send feedback on our drafts, help us track and resolve issues, summarize key www-style discussions on the CSSWG wiki and help us expand and improve the test suites.