<canvas id="myCanvas" width="520" height="520" style="border:1px solid black;"></canvas>

<script>
    var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");

    // Dibujar un rectángulo rojo
    ctx.fillStyle = "red";
    ctx.fillRect(50, 50, 200, 200);
</script>