<!DOCTYPE html>
<html>
<head>
<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, 100, 100);

ctx.beginPath();
ctx.rect(30, 30, 40, 40);
ctx.clip();

ctx.fillStyle = '#f00';

ctx.fillRect(0, 0, 100, 100);

</script>
</body></html>
