回到顶部-

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>
    <title>Javascript 回到顶部效果</title>
<style >
.box {
    width:1190px;
    margin:0 auto;
    }
#btn {
    width:40px;
    height:40px;
    position:fixed;
    left:50%;
    margin-left:590px;
    bottom:30px;
    background:url(http://img1.sycdn.imooc.com//535e0dc100010e9c00400080.jpg) no-repeat left top;
    margin:0 auto;
    }
#btn:hover {
    background:url(http://img1.sycdn.imooc.com//535e0dc100010e9c00400080.jpg) no-repeat 0 -40px;
    }
</style>
</head>
<body>

<div>
  <img src="http://img1.sycdn.imooc.com//535e0ce800015b7511902787.jpg" />
</div>
<a href="javascript:;" id="btn" title="回到顶部"></a>
</body>
</html>