<script>
  function RGB2Color(r,g,b) {
    return '#' + byte2Hex(r) + byte2Hex(g) + byte2Hex(b);
  }
  
  function byte2Hex(n) {
    var nybHexString="0123456789ABCDEF";
    return String(nybHexString.substr((n>>4) & 0x0F,1))+ 
           nybHexString.substr(n & 0x0F,1);
  }

  function rand(n) {
    return Math.floor((Math.random()*n));
  }

  function draw() {
	  var c=document.getElementById("canvas1");
	  var ctx=c.getContext("2d");
	  for (var i=0; i<h/2; i+=2) {
	    ctx.fillStyle = RGB2Color(rand(256),rand(256),rand(256));
	    ctx.fillRect(i,i,w-i*2,h-i*2);
	  } 
  }
  var h=window.innerHeight*0.9;
  var w=window.innerWidth*0.9;
  document.getElementById("canvas1").width=w;
  document.getElementById("canvas1").height=h;

  var v=setInterval(draw,200);

</script>