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.
custom-ident
is restricted only in cases where actually ambiguous parsing-wise. Specs referencing it should be clear about what is excluded.fill-box
and stroke-box
.