<meta charset="utf-8">
<canvas width="450" height="450"> </canvas>
<script>
var pantalla = document.querySelector("canvas");
var pincel = pantalla.getContext("2d");
pincel.fillStyle = "lightgrey"
pincel.fillRect(0,0,450,450);
pincel.fillStyle = "black";
pincel.beginPath();
pincel.moveTo(50, 50);
pincel.lineTo(50, 400);
pincel.lineTo(400, 400);
pincel.fill();
pincel.fillStyle = "yellow";
pincel.beginPath();
pincel.moveTo(100, 175);
pincel.lineTo(100, 350);
pincel.lineTo(275, 350);
pincel.fill();
</script>