<canvas width="600" height="400">
</canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.beginPath();
pincel.moveTo(50,50);
pincel.lineTo(50, 400);
pincel.lineTo(400, 400);
pincel.fill();
pincel.beginPath();
pincel.fillStyle = 'lightgray';
pincel.moveTo(100,175);
pincel.lineTo(100, 350);
pincel.lineTo(275, 350);
pincel.fill();
</script>
