This is a page from the Cascading Style Sheets Working Group Blog. Some other places to find information are the “current work” page, the www-style mailing list, the Future of CSS syndicator, and the issue list on Github.
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.
:matches() to :is() and deprecate :matches() in Safari and anywhere else using it (Issue #3258):blank (Issue #1967). The group will look at this in ~2 months once there’s data. If there’s no compat risk, Chrome is willing to change.:valid-within / :invalid-within pseudo-classes may need to move into Selectors 5 (Issue #3072). There’s also interest in investigating :has-child to see if that solves enough use cases to be worth doing.