Hello WebKit-dev, We've been working on CSS Containment performance for a while. Here is our summary of it, please take a look if you are interested. Thanks! https://docs.google.com/document/d/166CsE0BqOGur1UyeMqxC7OSijeH90R1Pdck54rLg... <https://docs.google.com/document/d/166CsE0BqOGur1UyeMqxC7OSijeH90R1Pdck54rLgZNU/edit?usp=sharing> Regarding the current state of CSS contain, the implement of layout, size, paint and style containment have been landed [1]. And the CSS contain runtime flag is enabled by default now [2]. \o/ [1] https://bugs.webkit.org/show_bug.cgi?id=172026 <https://bugs.webkit.org/show_bug.cgi?id=172026> [2] https://bugs.webkit.org/show_bug.cgi?id=234020 <https://bugs.webkit.org/show_bug.cgi?id=234020> Best, Rob and Cathie