<!DOCTYPE html>
<html>
<head>
    <script>
    function start() {
        var canvas = document.getElementById("cnv");
        var ctx = canvas.getContext("2d");
        ctx.fillStyle = "#ff0000";
        ctx.fillRect(0, 0, canvas.width, canvas.height);
    }
    </script>
</head>
<body onload="start()">

<div id="box" style="position: absolute; width: 300px; height: 200px; border-radius: 40px; overflow: hidden;">
<canvas id="cnv" width="400px" height="400px"></canvas>
</div>

</body>
</html>