CSS Selector Level 4

Selectors are patterns that match against elements in a tree, and as such form one of several technologies that can be used to select nodes in a document. Selectors have been optimized for use with HTML and XML, and are designed to be usable in performance-critical code. They are a core component of CSS (Cascading Style Sheets), which uses Selectors to bind style properties to elements in the document. Selectors Level 4 describes the selectors that already exist in [SELECT], and further introduces new selectors for CSS and other languages that may need them.

Related Articles:

https://drafts.csswg.org/selectors-4/