Browsers already do quite a bit of this -- for example one easy way to hurt performance is to have events that trigger on scrolling ( https://developer.chrome.c

Browsers already do quite a bit of this -- for example one easy way to hurt performance is to have events that trigger on scrolling ( https://developer.chrome.com/docs/devtools/rendering/performance/ https://developer.chrome.com/docs/devtools/rendering/perform... ). However, the current way to fix this is "don't do that". I'm not sure it would be faster for a page to announce it won't do things in advance, than the page just not do them, and then the existing fast paths get used.