根据li标签的个数动态生成span标签，并且点击span标签还会显示对应的li内容
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Title</title>
<link rel="stylesheet" href="css/3.css">
</head>
<body>
<ul id="box">
<li>item0</li>
<li>item1</li>
<li>item2</li>
<li>item3</li>
</ul>
<div id="spans">
</div>
</body>
<script src="js/js3.js"></script>
</html>
