<!DOCTYPE html>
<html>
<body>
<canvas id ="output" width="100" height="100" style="background: blue;"></canvas>
<script>
var canvas = document.getElementById('output');
var ctx = canvas.getContext('2d');

ctx.fillStyle = 'green';
ctx.fillRect(0, 0, 100, 100);

ctx.clearRect(40, 40, 20, 20);

ctx.strokeStyle = 'red';
ctx.lineWidth = 4;
ctx.strokeRect(20, 20, 60, 60);
</script>
</body>
</html>