请教一下，为什么最后两行较粗的js代码无法输出“PHP”呢？
<!DOCTYPE HTML>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>无标题文档</title>
</head>
<body>
<ul id="test">
<li>JavaScript</li>
<li>HTML</li>
</ul>
<script type="text/javascript">
var otest = document.getElementById("test");
var newNode=document.createElement("li");
newNode.innerHTML="PHP";
otest.appendChild(newNode);
var node=document.getElementsByTagName("li");
document.write(node.lastChild.innerHTML);
</script>
</body>
</html>
