We need a browser warning: "This site is using your computer heavily in a background task. Do you want to stop that?"

Doesn't Safari sort of already do that? "This tab is using significant power", or summat? I know I've seen that message, I just don't have a good repro.

Edge does, as well. It drops a warning in the middle of the screen, displays the resource-hogging tab, and asks whether you want to force-close the tab or wait.