页内跳转（锚点练习）
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Document</title>
<style>
body{
height: 2000px;
position: relative;
}
.a{
position: absolute;
bottom: 0px;
}
</style>
</head>
<body>
<a href="http://www.sina.com"><a href="#db">返回底部</a></a>
<hr />
<br />
<img src="images/1.jpg" alt="奥迪R8">
<p class="a" id="db"><a href="#last"><a href="#">返回顶部</a></a></p>
</body>
</html>
