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