<canvas class="myCanvas" width="600" height="400">
</canvas>
<script>
var canvas = document.querySelector("canvas");
var pincel = canvas.getContext("2d");
pincel.fillStyle = "back";
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>