Is there a special function for <th> anyway, or is it just the visual difference?
Please note there are two ways of specifying the table headers: TH cells (in which case you must specify a scope attribute) and the THEAD row group. The THEAD and TBODY groups are mandatory in HTML 4.x strict IIRC [...]
In XHTML 1.1 at least, thead, tfoot and tbody are optional [1]. In HTML 4.01, it seems to me that tbody at least is required, while thead and tfoot are optional [2].
1. http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_tablemodul... 2. http://www.w3.org/TR/html4/sgml/dtd.html