<canvas id="dest" height="100" width="100"></canvas>
<script>
var canvasWidth = canvasHeight = 100;
var destCanvas = document.getElementById('dest');
var destCtx = destCanvas.getContext('2d');
destCtx.fillStyle = 'red';
destCtx.fillRect(0, 0,  canvasWidth, canvasHeight);
destCtx.fillStyle = 'green';
destCtx.fillRect(0, 0, canvasWidth / 2, canvasHeight / 2);
destCtx.fillRect(canvasWidth / 2, canvasHeight / 2, canvasWidth / 4, canvasHeight / 4);
</script>