<meta charset="utf-8">
<canvas width="600" height="400" ></canvas>
<script>
var tela = document.querySelector ('canvas');
var pincel = tela.getContext ('2d');
//tela
pincel.fillStyle = "lightgrey"
pincel.fillRect (0, 0, 600, 400);
//esquadro1
pincel.fillStyle = 'black'
pincel.beginPath ();
pincel.moveTo (50, 50);
pincel.lineTo (50, 400);
pincel.lineTo (400, 400);
pincel.fill ();
//esquadro2
pincel.fillStyle = 'white'
pincel.beginPath ();
pincel.moveTo (100, 175);
pincel.lineTo (100, 350);
pincel.lineTo (275, 350);
pincel.fill ();
</script>