[tdrdt]

https://developer.mozilla.org/en-US/docs/Web/CSS/contain

The contain CSS property allows an author to indicate that an element and its contents are, as much as possible, independent of the rest of the document tree. This allows the browser to recalculate layout, style, paint, size, or any combination of them for a limited area of the DOM and not the entire page, leading to obvious performance benefits.

I wonder how much of this is browser specific because isn't this is all about implementation?

[shadowgovt]

Yes, it's extremely browser specific. This property is a rendering hint.