Meu creeper ficou dessa forma. Ficou meio confuso, mas funcionou.
<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext('2d');
pincel.fillStyle = "darkgreen";
pincel.fillRect(0, 0, 350, 300);
pincel.fillStyle = "black";
pincel.fillRect(100, 190, 40, 110);
pincel.fillRect(210, 190, 40, 110);
pincel.fillRect(140, 140, 70, 100);
pincel.fillRect(50, 50, 90, 90);
pincel.fillRect(210, 50, 90, 90);
</script>
