'Twas brillig, and the slithy toves did gyre and gimble in the wabe

How on earth have you not known about Firebug lite before... Hell, I actually assumed that project was long gone. It started as a way of getting dev tools into IE (IE 8 was the first to include dev tools if memory serves).

We used it at work 6 year ago...

That said, Safari seems like an interesting use of it. Quite like the trick Chrome are doing for mobile browser dev where you like your desktop copy of the mobile and use it on your desktop to interact with the phones copy.