返回的前一节点问题
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>nextSibling</title>
</head>
<body>
<ul id="u1">
<li id="a">javascript</li>
<li id="b">jquery</li>
<li id="c">html</li>
</ul>
<ul id="u2">
<li id="d">css3</li>
<li id="e">php</li>
<li id="f">java</li>
</ul>
<script type="text/javascript">
function get_previousSibling(n){
var z=n.previousSibling;
while(z && z.nodeType!=1){
z=z.previousSibling;
}
return x;
}
var a=document.getElementsByTagName("li")[3];
document.write("<br/>"+a.nodeName);
document.write("=");
document.write(a.innerHTML);
var b=get_previousSibling(a);
if(b!=null){
document.write("<br/>"+b.nodeName);
document.write("=");
document.write(b.innerHTML);
}else{
document.write("<br/>已经是第一个节点")
}
</script>
</body>
</html>为啥返回的是“JavaScript”而不是“html”，这和父节点有关系吗？如果有，规则是什么？
麻烦各位大神帮忙看下
