直接上代码啦：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>Untitled</title>
</head>
<body>
<script type="text/javascript">
function foo() {
alert("document.body.parentNode.previousSibling.tagName\n" + document.body.parentNode.previousSibling.tagName);
alert("document.body.parentNode.parentNode.firstChild.tagName\n" + document.body.parentNode.parentNode.firstChild.tagName);
alert("document.body.parentNode.parentNode.firstChild.nodeName\n" + document.body.parentNode.parentNode.firstChild.nodeName);
alert("document.body.parentNode.parentNode.firstChild.nodeValue\n" + document.body.parentNode.parentNode.firstChild.nodeValue);
alert("document.body.parentNode.parentNode.lastChild.tagName\n" + document.body.parentNode.parentNode.lastChild.tagName);
alert("document.body.parentNode.parentNode.lastChild.nodeName\n" + document.body.parentNode.parentNode.lastChild.nodeName);
}
function redoctypeme(which) {
alert("before\n" + document.body.parentNode.parentNode.firstChild.nodeValue);
if(which == 1) {
document.body.parentNode.parentNode.firstChild.nodeValue = "DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01//EN\" \"http://www.w3.org/TR/html4/strict.dtd\"";
}
else {
document.body.parentNode.parentNode.firstChild.nodeValue = "DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\"";
}
alert("after\n" + document.body.parentNode.parentNode.firstChild.nodeValue);
foobar = document.body.innerHTML;
alert("empty");
document.body.innerHTML = "";
alert("reset");
document.body.innerHTML = foobar;
}
</script>
<a href="javascript: foo()">stuff</a>
<a href="javascript: redoctypeme('1')">doctype 1</a>
<a href="javascript: redoctypeme('2')">doctype 2</a>
<hr>
<div style="background: #0c0; 200px; color: #fff;">200px</div>
<div style="background: #c00; 200px; padding: 20px; border: 30px solid #00c;"> </div>
<div style="background: #0c0; 300px; color: #fff;">300px</div>
</body>
</html>