<!DOCTYPE html>
<html>
<body>
<canvas id='output' width='100' height='100'></canvas>
<script>
var aCanvas = document.getElementById('output');
var ctx = aCanvas.getContext('2d');
ctx.fillStyle = 'green';
ctx.fillRect(20, 20, 60, 60);
ctx.clearRect(25, 25, 50, 50);
</script>
</body>
</html>