<!DOCTYPE html>
<html>
<head>
</head>
<body>
<canvas id="canvas" width="400" height="400"></canvas>
<script>
async function start() {
    const canvas = document.getElementById("canvas");
    const context = canvas.getContext("2d");
    context.fillStyle = "blue";
    context.fillRect(0, 0, 400, 400);
    context.fillStyle = "white";
    context.fillRect(100, 100, 200, 200);
}
window.addEventListener("load", start);
</script>
</body>
</html>