Outside the mac world, "F11" is a pretty much universal "toggle full screen" binding. Chrome at least supports with on-screen help when you enter full screen mode.
As you said, F11 is universal for toggling on off, but it's not a good solution when there may be multiple elements on a page which might fight for fullscreen requests. For instance, if I have 4 videos on a page, and F11 is pressed, which video goes full screen? In the context of web browsing, where the user is choosing which elements to launch full screen, the ESC button really should be standardized.
I don't follow. Obviously you can only toggle to full screen for something unambiguous (i.e. the whole browser page). I don't see how pressing ESC helps the user decide which of the videos to launch.
But once you're full screen, F11 does indeed get you back to the original state. So I still don't see why ESC is needed.
The main reason is that you probably got into full-screen by pressing a button, not the F11 key, and now you just want a "get me out of this" key which escape is better for.
