Diseñando una escuadra
<canvas width="500" height="400"> </canvas>
<script>
var panel = document.querySelector("canvas");
var pincel = panel.getContext("2d");
//Creating external rectangle
pincel.beginPath();
pincel.moveTo(50,50);
pincel.lineTo(50,400);
pincel.lineTo(400,400);
pincel.fill();
//Creating inside rectangle
pincel.fillStyle = "white"
pincel.beginPath();
pincel.moveTo(100,175);
pincel.lineTo(100,350);
pincel.lineTo(275,350);
pincel.fill();
</script>