<!DOCTYPE html>
<script>
window.onload = function () {
    var ctx = document.getElementById('c').getContext('2d');
    ctx.fillStyle = 'green';
    ctx.fillRect(0, 0, 100, 100);
    ctx.fillStyle = 'blue';
    ctx.fillRect(10, 10, 80, 80);
    ctx.fillStyle = 'red';
    ctx.fillRect(20, 20, 60, 60);
}
</script>
<p>The canvas below should contain green, blue and red nested squares, all centered with respect to each other.</p>
<canvas id="c" width="100" height="100"></canvas>