<td class="dataTableHeadingContent" align="left" valign="top">
<?php echo (($_GET['list_order']=='lastname' or $_GET['list_order']=='lastname-desc') ? '<span class="SortOrderHeader">' . TABLE_HEADING_LASTNAME . '</span>' : TABLE_HEADING_LASTNAME); ?><br>
<a href="<?php echo zen_href_link(basename($PHP_SELF), zen_get_all_get_params(array('list_order','page')) . 'list_order=lastname', 'NONSSL'); ?>"><?php echo ($_GET['list_order']=='lastname' ? '<span class="SortOrderHeader">Asc</span>' : '<span class="SortOrderHeaderLink">Asc</b>'); ?></a>&nbsp;

All these spans are closed with /b and not /span