Absolutely lovely way of saving space. But we're blurring the line between browser and content now, and this is going to confuse less technologically gifted people if not done properly.

Furthermore, are other sites able to use this feature as well?

I'm personally worried that the current trend of browsers deviating far from a "standard" look will create problems for those who are not as prone to tinkering as we are. For them, things seemingly disappear and (re-?)appear at random and apart from the back button and the address bar there's really not much in the way of consistency anymore.

Even Explorer windows in Windows do this by hiding the menu bar unless you press Alt. It's not intuitive; "our kind" is very much an exception with our ability to find these things after the fact.