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 det
As you have discovered, browser compatibility is a big drawback. After all, this is something very new.
However, since you're working in JavaScript, you have far more options available to you than if you were just using CSS.
For example:
if( window.innerHeight == screen.height) {
// browser is fullscreen
}
You can also check for some slightly more loose comparisons:
if( (screen.availHeight || screen.height-30) <= window.innerHeight) {
// browser is almost certainly fullscreen
}