var fscr = document.documentElement;
     function FullScreen() {
       if(fscr.requestFullScreen) {
        fscr.requestFullScreen();
       } else if(fscr.mozRequestFullScreen) {
        fscr.mozRequestFullScreen();
       } else if(fscr.webkitRequestFullscreen) {
        fscr.webkitRequestFullScreen();
       } else if(fscr.msRequestFullscreen) {
        fscr.msRequestFullscreen();
       }
}

     function ExitFullScreen() {
       if (document.exitFullscreen) {
        document.exitFullscreen();
       } else if (document.mozCancelFullScreen) {
        document.mozCancelFullScreen();
       } else if (document.webkitCancelFullScreen) {
        document.webkitCancelFullScreen();
       } else if (document.msExitFullscreen) {
        document.msExitFullscreen();
       }
}

<button onmouseover='this.style.cursor="pointer"' onclick="FullScreen();">Полноэкранный режим</button>
<button onmouseover='this.style.cursor="pointer"' onclick="ExitFullScreen();">Обычный режим</button>

:-ms-fullscreen {   
width: 100%; /*иначе IE не растянет на всю ширину страницы*/
}