Okay, thanks for your answer.
Hmm.. How about a plugin? Maybe can work with this way. I don't know, to activate a "fake" action, whatever.
Maybe inside "Browser" plugin there is a line for this, to modify....... Just a thought..
It's a browser limitation nothing can be done about that, it's all up to the browser vendors as far as I know.
You could do a classic startup menu where you'd ask the user to "press any key" or "tap the screen" to continue.
If you want my opinion about this, I would never try to force fullscreen mode.
I would rather start in windowed mode and add an option to toggle it on demand.