HTML is not necessarily case sensitive, you can use upper or lowercase tags in HTML 5, and perhaps others.  With that in mind, the tags should be normalized when querying what an element is (hr == HR == hR == Hr).  Uppercase was chosen:<p><a href="http://www.whatwg.org/specs/web-apps/current-work/multipage/apis-in-html-documents.html#apis-in-html-documents" rel="nofollow">http://www.whatwg.org/specs/web-apps/current-work/multipage/...</a><p>"These attributes must return element names converted to ASCII uppercase, regardless of the case with which they were created."