<canvas>width= "600" heigth= "400"></canvas>
<script>
    var tela = document.querySelector('canvas');
    var pincel = tela.getContext('2d');
    pincel.fillStyle= "black";
    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> 
             
            