<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="White";
    pincel.beginPath();
    pincel.moveTo(100, 175);
    pincel.lineTo(100, 350);
    pincel.lineTo(275, 350);
    pincel.fill();
</script> 
            