In this article, I’m going to introduce a CSS Specification that has just become a W3C Recommendation. The CSS Containment Specification defines a single property, contain, and it can help you to explain to the browser which parts of your layout are independent and will not need recalculating if some other part of the layout changes.
While this property exists for performance optimization reasons, it can also affect the layout of your page.
https://www.smashingmagazine.com/2019/12/browsers-containment-css-contain-property/