The CSS Working Group has published an updated Candidate 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.

CSS is a language for describing the rendering of structured documents (such as HTML and XML) on screen, on paper, etc.

Original source: https://www.w3.org/blog/news/archives/7731