<!DOCTYPE html>
<html>
<body>
<p>A green square should appear below this text.</p>
<canvas id="c" width="100" height="100"></canvas>
<script>
window.onload = function() {
    var canvas = document.getElementById("c")
    var ctx = canvas.getContext("2d")
    ctx.fillStyle = 'green';
    ctx.fillRect(0, 0, 100, 100);
}
</script>
</body>
</html>