<canvas width = "600" height = "400" ></canvas>
<script>
var tela = document.querySelector("canvas");
var pincel = tela.getContext("2d");
pincel.beginPath();
pincel.moveTo(50,50);
pincel.lineTo(50,400);
pincel.lineTo(400,400);
pincel.fill();
pincel.fillStyle = "green";
pincel.beginPath();
pincel.moveTo(100,175);
pincel.lineTo(100,350);
pincel.lineTo(275,350);
pincel.fill();
</script>
