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.
#123abc
to be a valid ID selector, to match HTML5. Need to collect data on this.
:nth-child()
/etc don’t require a parent: they’re based on siblings. This is a change from Selectors Level 3 REC.
:matches()
to the specificity of the actual matched selector.