webgl_2_clear_screen.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8"/>
    <title>clear</title>
</head>
<body>

<canvas id="canvas" width="400" height="400">
    Please use a browser that supports "canvas"
</canvas>

<script>
    var canvas = document.getElementById("canvas");
    var gl = canvas.getContext('webgl');

    gl.clearColor(0.0, 0.0, 0.0, 1.0);
    gl.clear(gl.COLOR_BUFFER_BIT);

</script>
</body>
</html>