<!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.moveTo(100, 30);
ctx.moveTo(30, 30);
ctx.lineTo(30, 70);
ctx.lineTo(70, 70);
ctx.lineTo(70, 30);
ctx.closePath();
ctx.clip();
ctx.fillStyle = '#f00';
ctx.fillRect(0, 0, 100, 100);
</script>
</body></html>