+4  A: 

Like this:

alert("Please press F11");

There's no way to make an existing window completely fullscreen in Javascript.

The closest you can get is to open a popup window and pray that it doesn't get blocked by popup blockers, like this:

open("http://google.com", null, 
  "height=" + screen.availHeight + ",width=" + screen.availWidth + ",status=no,toolbar=no,menubar=no,location=no")
SLaks
+1 for the alert.
peirix
Are you so sure that it's impossible to implement F11 effect?
Yes. I am sure.
SLaks
yes, it shouldn't be allowed
Luca Matteis