<meta charset="utf-8">
<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = "white";
pincel.fillRect(0,0,600,400);//cria retangulo cinza do tamanho do canvas
pincel.beginPath();
pincel.fillStyle="black";
pincel.moveTo(50, 50);
pincel.lineTo(50, 400);
pincel.lineTo(400,400);
pincel.fill();
pincel.beginPath();
pincel.fillStyle="white";
pincel.moveTo(100, 175);
pincel.lineTo(100, 350);
pincel.lineTo(275,350);
pincel.fill();
</script>
