Modern desktop version of IE 10 is always fullscreen.

There is a living specification for :fullscreen pseudo-class on W3

But when I tried to detect fullscreen with jQuery version 1.9.x and 2.x:

$(document).is(":fullscreen") 

it threw this error:

Syntax error, unrecognized expression: fullscreen

Questions:

1. Is it because jQuery doesn't recognize this standard yet or IE10?

2. What is the legacy way of checking fullscreen mode? I am expecting following results:

   function IsFullScreen() {
        /* Retrun TRUE */
        /* If UA exists in classic desktop and not in full screen  */
        /* Else return FALSE */
   }
   

3. Can we do it without browser sniffing?