[CSS] スクロールしても横固定のメニュー

<!DOCTYPE html>
<html lang="en">
<head>
	<meta charset="UTF-8">
	<title>Document</title>
</head>
<body>
<header id="header"></header>
<main id="main">
	<h2>MAIN BLOCK</h2>
</main>

<aside id="sub">
	<h2>Menu</h2>
	<ul>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
	</ul>
	<h2>Menu</h2>
	<ul>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
	</ul>
	<h2>Menu</h2>
	<ul>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
		<li>Subject</li>
	</ul>
</aside>
</body>
</html>
body {
	margin: 0;
	padding: 0;
}
#header {
	box-sizing: border-box;
	background: #ccc;
	height: 50px;
}
#main {
	box-sizing: border-box;
	margin-left: 220px;
	padding: 20px 40px;
}
#sub {
	box-sizing: border-box;
	top: 50px;
	height: 100%;
	width: 220px;
	position: fixed;
	overflow: auto;
	background: #eee;
	padding: 20px;
}

OK, keep goin
出掛けよう