节点值为null
<body>
<ul>
<li>javascript</li>
<li>HTML/CSS</li>
<li>jQuery</li>
</ul>
<script type="text/javascript">
var a = document.getElementsByTagName("li");
for(var i = 0; i < a.length; i++){
document.write("" + a[i].innerHTML + "<br>");
document.write("元素节点是:" + a[i].nodeName + "<br>");
document.write("节点值是:" + a[i].nodeValue + "<br>");
document.write("节点类型是:" + a[i].nodeType + "<br>");
}
</script>
</body>