It's a bit unclear if the returned value is uppercase or not. So far I've yet to come across a case when it's not in UPPERCASE. Also according to the the specification:
The HTML DOM returns the tagName of an HTML element in the canonical uppercase form, regardless of the case in the source HTML document.
It's been two years, my experience is the same as Leidegre's. If the page is actually correct, it would help to see an example. Page Notes 13 July 2010