The page does not open in full screen mode [duplicate] – Blancer.com Tutorials and projects

I would like to open page in full screen mode after ‘onload’. I use JSF wit primefaces. This is JavaScript that I use for it:

function cancelFullScreen(el) {
  var requestMethod = el.cancelFullScreen || el.webkitCancelFullScreen || el.mozCancelFullScreen || el.exitFullscreen;
  if (requestMethod) { // cancel full screen.
    requestMethod.call(el);
  } else if (typeof window.ActiveXObject !== "undefined") {
    var wscript = new ActiveXObject("WScript.Shell");
    if (wscript !== null) {
      wscript.SendKeys("{F11}");
    }
  }
}

function requestFullScreen(el) {
  var requestMethod = el.requestFullScreen || el.webkitRequestFullScreen || el.mozRequestFullScreen || el.msRequestFullScreen;
  if (requestMethod) { // Native full screen.
    requestMethod.call(el);
  } else if (typeof window.ActiveXObject !== "undefined") {
    var wscript = new ActiveXObject("WScript.Shell");
    if (wscript !== null) {
      wscript.SendKeys("{F11}");
    }
  }
  return false;
}

Generally it works fine when I add this action for a button or different event like example:

<body onmousedown="requestFullScreen(document.body);return false;">

But it does work when I try to run it when page open:

<body  onunload="requestFullScreen(document.body);return false;">

Where am I making a mistake?