CSS Containment Module Level 1 is a W3C Proposed Recommendation; Level 2 is a First Public Working Draft
The CSS Working Group has published a Proposed Recommendation of CSS Containment Module Level 1. This CSS module describes the contain property, which indicates that the element’s subtree is independent of the rest of the page. This enables heavy optimizations by user agents when used well. Comments are welcome through 13 November 2019.
CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.