Pulseaudio also allows you to alter the volume of apps using the device chooser and it's already the default for many linux distros including ubuntu. In fact ts also allows you to send the sound from an individual app (say firefox up to its eyes in noisy adverts) over the network to another computer for them to hear it ;)
"You know what, for once I can't think of anything bad about an Apple idea. Wonder if they'll ever make it."
err what about the fact they are trying to patent the idea rather than releasing it and benefiting everyone
"Apple wants to give you the opportunity to selectively block web-based audio while allowing you to listen to any other audio source of your choosing."
No they want to give apple users or more generously safari users the opportunity and prevent anyone else.