2
To be more helpfully here are some lines for one of the
causes of the non-compliance to XHTML (which is the single & instead of &)
in modules/newbb/include/notification.inc.php line 56
in modules/newbb/delete.php line 52, 57, 74
in modules/newbb/edit.php line 64, 83, 101
in modules/newbb/post.php line 126, 142, 182
in modules/newbb/reply.php line 43, 47, 71, 91
Hope this is helpful for one of the developers.