You mean my browser still doesn't use enough CPU? I don't want sites that aren't in focus to do anything.
Seriously, web devs need to learn to respect my battery and electricity bill more.
It's not about doing stuff in a tab that isn't in focus. "Idle" refers to the main thread idling in the page you're currently using, so it's about controlling the execution order and priority of JS tasks. E.g. on Google Maps you give priority to rendering the visible map tiles while deferring preloading the surrounding non-visible tiles until the main thread is unblocked (and thus idling).