My friend Barak Sh says:
… probably around 99.9% of the world’s population already knows that you should “press ESC to exit full screen mode” in YouTube. Isn’t it time to stop showing this extremely annoying message?
I would probably disagree and say that assuming such a thing about 99.9% is too bold, but…
I’m teaching my father (62) to use the Internet. I had to explain him many times what is spam, what are tabs in the browser, what are links, how to write a subject in an email and how to save email attachments to a local folder. Very simple things, but he’s still not so sure about most of them.
But he immediately understood that pressing ESC exits full-screen mode in YouTube! And he applied this knowledge to sport5.co.il (nonstandard).
So he’s probably not too far off the mark.
Aharoni in Unicode 
Actually, pressing ESC doesn’t come intuitively for me to leave full-screen mode, and I’ve been using computers daily since the late 1980s. In Quicktime for Mac, I enter and leave full-screen mode by pressing CMD-F. In VLC for Mac, I enter and leave full-screen mode by the same method or by double clicking the movie window. I just checked and was quite surprised to learn that ESC exits full-screen mode in both programs.
Steven wright once said that “42.7% of all statistics are made up on the spot.”. That’s why I’m cautious about flinging those “99%”‘s, “99.9%”‘s and their ilk.