Do you want to know how the CSS WG works? Fantasai has written about:csswg, An Inside View of the CSS Working Group at W3C.
CSS is both a standard and a technology in development. Knowing which part of CSS is finished and which part you cannot use yet is a challenge. It’s been like that for more than a dozen years and is likely to stay that way for a couple more: the standard part is slowly growing, the part still in development has been growing as well, but should eventually diminish.
The CSS working group has written a document for implementers to explain the process of development and the current state of CSS. That document is called the CSS Snapshot. The working group intends to update it about every two years, or more often if needed.
But even if you are not an implementer, but only trying to follow the development of CSS and discuss it, e.g., on the <email@example.com> mailing list, it may be useful to read the snapshot.
You can also read an alternative version, without the current state of CSS, but with a slightly expanded text describing the CSS process and its various types of documents.