HTML:
<body onload="load()"> <canvas id="canvas_back"></canvas> ... </body>
CSS:
#canvas_back {
  position: fixed;
  top: 0px;
  left: 0px;
  z-index: -1;
}
JavaScript:
var c, w, h;
function load() {
  var canvas = document.getElementById("canvas_back");
  c = canvas.getContext("2d");
  window.addEventListener('resize', resize, false);
  resize();
}
function resize() {
  w = canvas.width = window.innerWidth;
  h = canvas.height = window.innerHeight;
  draw();
}
function draw() {
  // use c, w and h to draw the canvas
  ...
  requestAnimationFrame(draw);
}
Demo 1
done_