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>
</head>
<body>

<picture>
    <source srcset="https://www.cainiaoplay.com/wp-content/uploads/2015/06/img_smallflower.jpg" media="(max-width: 400px)">
    <source srcset="https://www.cainiaoplay.com/wp-content/uploads/2015/06/img_flowers.jpg">
    <img src="https://www.cainiaoplay.com/wp-content/uploads/2015/06/img_flowers.jpg" alt="Flowers" style="width:auto;">
</picture>

<p>调整浏览器宽度和高度，背景在宽度小于 400px 时将改变。 </p>

</body>
</html>
源代码

动行结果
