<head>
    <meta name="viewport" content="width=device-width">
    <style>

    canvas {
        width: 100px;
        height: 100px;
        background-color: red;
        box-shadow: 10px 10px 0px blue;
    }

    </style>
</head>
<canvas>
<script type="text/javascript">

    var canvas = document.querySelector("canvas");
    canvas.width = canvas.clientWidth * window.devicePixelRatio;
    canvas.height = canvas.clientHeight * window.devicePixelRatio;

    var gl = canvas.getContext("webgl");
    gl.clearColor(0, 0, 0, 1);
    gl.clear(gl.COLOR_BUFFER_BIT);

</script>
</canvas>