CSS has a containment property whichs sole purpose is to improve rendering performance.
So, first, we develop an abstraction so developers don't have to worry about implementation details like performance. Then, as soon as we realise that our abstractions are (obviously) dog slow, we add more stuff to make them fast again and have developers worry about that instead.
At this point, we might as well ship websites as compiled binaries.
@MerriNet Not saying you're wrong, but I just went by what the MDN said: "The aim of the CSS containment module is to improve performance of web pages by allowing the browser to isolate a subtree of the page from the rest of the page."
What I said is that these performance features have existed in CSS for 20+ years. You did need to know to use overflow: hidden; if you wanted a well performing collapse/expand animation.
The performance worry has always been there. You worry about perf no matter what you use or do.
There are far more idiotic things happening on the web space. Just look at React. It is the best dev time waster to perf and size optimizations, and people still keep using it.