<canvas id="canvas" width="100" height="100"></canvas>
<script>
var canvas = document.getElementById('canvas');
var ctx = canvas.getContext('2d');
ctx.fillStyle = '#0f0';
ctx.fillRect(0, 0, canvas.width, canvas.height);
ctx.fillStyle = '#00f';
ctx.fillRect(40, 40, 20, 20);
</script>