The CSS Working Group has published an updated Working Draft of Selectors Level 4. Selectors are patterns that match against elements in a tree, and are used most famously in CSS style rules as well as various Web platform APIs.
This long overdue update adds quite a few new pseudo-classes: :defined, :modal, :fullscreen, :picture-in-picture, :muted, :volume-locked, :seeking, :buffering, :stalled, :autofill, and :user-valid. It also adjusts arsing rules for most pseudo-classes such as :is() that accept selectors (but not :not()) to ignore invalid selectors in their arguments, preventing these from invalidating the entire selector. Changes since the last Working Draft are listed in the Changes section.
Please send feedback by either filing an issue in GitHub (preferable) or sending mail to the (archived) public mailing list www-style@w3.org with the spec code ([selectors-4]) and your comment topic in the subject line. (Alternatively, you can email one of the editors and ask them to forward your comment.)