用document.getElementsByTagName("li")获得的Li元素出现以下现象的原因？
<!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_nextSibling(n){
var x=n.nextSibling;
while (x && x.nodeType!=1){
x=x.nextSibling;
}
return x;
}
function get_previousSibling(n){
var x=n.previousSibling;
while(x && x.nodeType!=1){
x=x.previousSibling;
}
return x;
}
var x=document.getElementsByTagName("li")[1];
document.write(x.nodeName);
document.write(" = ");
document.write(x.innerHTML);
var y=get_nextSibling(x);
if(y!=null){
document.write("<br />nextsibling: ");
document.write(y.nodeName);
document.write(" = ");
document.write(y.innerHTML);
}else{
document.write("<br>已经是最后一个节点");
}
var z=get_previousSibling(x);
if(z!=null){
document.write("<br />previoussibling: ");
document.write(z.nodeName);
document.write(" = ");
document.write(z.innerHTML+"<br />");
}else{
document.write("<br>已经是第一个节点");
}
</script>
</body>
</html>当 var x=document.getElementsByTagName("li")[2];时：
当当 var x=document.getElementsByTagName("li")[3];时：
当当 var x=document.getElementsByTagName("li")[4];时：
请问这是什么原因？
