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Browsers can do that, scrolling is cheap (unless you highjack it with JS, but that's another story). However, if change the layout / content of some element in the page, making it bigger, or smaller, or positioned differently, etc, that has an impact on its parents, recursively. So browsers mark subtrees as dirty when something changes, and work their way back up. In certain cases, there are changes that cannot have an effect on the parents, so you should be able to stop the recursion. And browsers typically are reasonably smart about that. However, there are a surprisingly large number of cases where checking whether or not the parents could possibly be affected by the change is an expensive operation, so you're better off relaying out your parents (recursively) rather than checking if you could safely skip doing it. Css-contain changes the rules of how css works, to eliminate these expensive to check cases. contain:content makes a few useful things impossible. contain:strict is even harsher, as parents' size cannot depend on children's size. But if you happen to have an element that can be set up to fit within these constraints, turning containment on on the right element lets the browser know that a whole bunch of optimizations are safe without checking for preconditions. |