// https://developer.mozilla.org/en-US/docs/Web/API/Fullscreen_API
function toggleFullscreen(el, fallback){
    
    var body = document.body
    
    // fullscreen API not supported, use fallback if given
    if( !body.requestFullscreen &&
        !body.webkitRequestFullScreen &&
        !body.msRequestFullscreen &&
        !body.mozRequestFullScreen
    ){
        fallback && fallback()
        return
    }
    
    // if NOT already in full screen, go full screen now
    if( !document.fullscreenElement &&
        !document.webkitFullscreenElement &&
        !document.msFullscreenElement &&
        !document.mozFullScreenElement
    ){    
        if (el.requestFullscreen)
            el.requestFullscreen();
        else if (el.msRequestFullscreen)
            el.msRequestFullscreen();
        else if (el.mozRequestFullScreen)
            el.mozRequestFullScreen();
        else if (el.webkitRequestFullscreen)
            el.webkitRequestFullscreen();
            
    }else{
        if (document.exitFullscreen)
            document.exitFullscreen();
        else if (document.msExitFullscreen)
            document.msExitFullscreen();
        else if (document.mozCancelFullScreen)
            document.mozCancelFullScreen();
        else if (document.webkitExitFullscreen)
            document.webkitExitFullscreen();
    }
}