You're being a bit disingenuous by not mentioning the inbuilt protections that the HTML5 Fullscreen API offers.
"Also, any alphanumeric keyboard input while in full-screen mode causes a warning message to appear; this is done to help guard against phishing attacks. The following keys are the only ones that don't cause this warning message to appear (...)"
That documentation is out-of-date. There were no warnings on keyboard input in Firefox or Chrome. I went fullscreen on a Facebook photo and was able to leave a comment without any issues.
Safari, on the other hand, appears to prevent keyboard input, which I just recently found out.
"Also, any alphanumeric keyboard input while in full-screen mode causes a warning message to appear; this is done to help guard against phishing attacks. The following keys are the only ones that don't cause this warning message to appear (...)"
(https://developer.mozilla.org/en-US/docs/DOM/Using_full-scre...)
The article and demo are nice though. Good work.