The latest version of Safari shows no warning on fullscreen, making users very vulnerable. The only indication is a short, half-second animation (it's much shorter than the usual OS X fullscreen animation). After that, there's no indication that you're in fullscreen mode.
Does it also disable using any kind of keyboard event? Because if so, that cuts out a ton of legitimate use cases for full screen. If not, it just makes the vulnerability slightly more of a pain to exploit.