<canvas width="400" height="400"> </canvas>
<script>
var pantalla = document.querySelector("canvas");
var pincel = pantalla.getContext("2d");
pincel.fillStyle = "white"
pincel.fillRect(0,0,400,400);
pincel.fillStyle = "black";
pincel.beginPath();
pincel.moveTo(0,400);
pincel.lineTo(0,0);
pincel.lineTo(400,400);
pincel.fill();
pincel.fillStyle = "white"
pincel.beginPath();
pincel.moveTo(35,350);
pincel.lineTo(35,100);
pincel.lineTo(280,350);
pincel.fill();
</script>