Maybe i didn't make myself clear. Sorry, I'm not a native speaker.

I asked how hard it is to make a game, specifically html5 one, that occupies the whole area of a browser window.

Secondly, f11 doesn't work for this specific site.

It is quite easy, you just need to specify the size in the cs (or set it with js).

Why didn't you do it, then?

Didn't think it was necessary.