<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<style>
			.box{
				width:100%;
				height:100px;
				max-width:640px;
				min-width:320px;
				position:fixed;
				top:0;
				left:50%;
				border:1px solid red;
				transform:translateX(-50%);
			}
		</style>
	</head>
	<body>
		<div class="box"></div>
	</body>
</html>