关于document.write("p1元素的属性值为："+p1.className);为什么不显示属性值，请问错在哪里？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=gb2312">
<title>className属性</title>
<style>
body{ font-size:16px;}
.one{
border:1px solid #eee;
width:230px;
height:50px;
background:#ccc;
color:red;
}
.two{
border:1px solid #eee;
width:230px;
height:50px;
background:#ccc;
color:green;
</style>
</head>
<body>
<p id="p1" > JavaScript使网页显示动态效果并实现与用户交互功能。</p>
<input type="button" value="添加样式" onclick="add()"/>
<br>
<script type="text/javascript">
var p1 = document.getElementById("p1");
document.write("p1元素的属性值为："+p1.className);
function add()
{
p1.className="two";
}
</script>
</body>
</html>
