<canvas id="canvas"></canvas>
<script type="text/JavaScript">
var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;
ctx.globalAlpha = 1.0;
ctx.fillStyle = "blue";
ctx.fillRect(10, 10, 100, 100);
ctx.fillStyle = "red";
ctx.fillRect(40, 40, 100, 100);
</script>