<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0"/>
<style>
body {
background-color:lightgreen;
}
@media only screen and (max-width: 500px) {
body {
background-color:lightblue;
}
}
</style>
</head>
<body>
<p>重置浏览器大小，当文档的宽度小于 500 像素，背景会变为浅蓝色，否则为浅绿色。</p>
</body>
</html>
