为什么多个span标签在一起，显示会重叠
<!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">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style>
div {
width:700px;
height:30px;
background:#fff;
position:relative;
}
div span {
width:40px;
height:30px;
position:absolute;
}
</style>
<script>
function()
{
}
</script>
</head>
<body>
<div id="pag">
<span class="aa" "><a href="http://www.baidu.com" >Itmem0</a></span>
<span class="aa" "><a href="http://www.baidu.com" >Itmem0</a></span>
<span class="aa" "><a href="http://www.baidu.com" >Itmem0</a></span>
<span class="aa" "><a href="http://www.baidu.com" >Itmem0</a></span>
<span class="aa" "><a href="http://www.baidu.com" >Itmem0</a></span>
</div>
</body>
</html>
