在线运行代码 - 菜鸟教程

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta charset="utf-8"> 
<title>菜鸟教程(cainiaoplay.com)</title>
<style>/* For device width smaller than 400px: */
body {
    background-repeat: no-repeat;
    background-image: url('https://www.cainiaoplay.com/wp-content/uploads/2015/06/img_smallflower.jpg');
}

/* For device width 400px and larger: */
@media only screen and (min-device-width: 400px) {
    body { 
       background-image: url('https://www.cainiaoplay.com/wp-content/uploads/2015/06/img_flowers.jpg');
    }
}
</style>
</head>
<body>

</body>
</html>
源代码

动行结果
