这个页面怎么制作,用textarea里面输入不了有效代码


<!DOCTYPE HTML>
<html>
<body>
<style>
*{padding: 0;margin:0;}
ul{width:200px;background:#f2f2f2;height:200px;overflow-y:auto;}
li{list-style:none;line-height:30px;background:green;margin:10px;color:#fff;text-align:center;}
</style>
<ul>
<li>w3school</li>
<li>w3school</li>
<li>w3school</li>
<li>w3school</li>
<li>w3school</li>
</ul>
</body>
</html>