.tagName evaluates to the name of the HTML 4 element, and names are written all-caps in the HTML 4 and older specifications.  All-caps is convenient because you can write about, for example, the A element by just typing its name.  The standards committees renamed them to lowercase for XHTML and HTML 5, because writing about 'a' elements is <i>ever</i> so much more consistent. Morons.<p>P.S. In HTML 4 and older, tags are case insensitive.  And practical browsers will accept both cases for the foreseeable future.