The actual page source is very funny. In the middle of several thousand lines of drivel is the actual content.
At line 1732:
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN" "http://www.w3.org/TR/REC-html40/loose.dtd">
<html>
<body>
<p>Here are some things that are built into the HTML and CSS standards that you probably didn't know about.
Or maybe you did. Congratulations.</p>
... (about 25 lines with all the real content)
</body>
</html>
Yes, there's an entire HTML document in the middle of the document. Which is just wrong. Then there's another thousand lines of HTML. And yes, there's Javascript.
If you cut out the inner document and put it in a file, you can open it and see the useful content.
Looks a bit like server side includes done wrong. Accidentally include the head/body/non content tags in the include, import them into the main file and well, you end up with duplicates.
Firefox at least interprets that as an ordinary include, rather than as a subdocument like a frame or an iframe. You'd think that <html> would start a new context, with the defaults for a new page. But it doesn't.
If you cut out the inner document and put it in a file, you can open it and see the useful content.