firstly, there's always some security and plenty of privacy mischief around focus.
Oh, how so?
i've actually played with this in the firefox debugger and it essentially appears feasible so really hope this feature comes oneday - or i finally get some time to look into making an addon for it
that's cool, yes a browser should stop using resources when you stop using it ( minimize it ), or using that particular tab by making it inactive, chromium based browsers behave like that if I'm not mistaken
That would also look cool, aesthetically speaking