In fckeditor.module line 59
Original:

$output = '<script language="JavaScript1.2" src="modules/fckeditor/lib/fckeditor.js" ></script>\n';

Fixed:

$output = '<script language="JavaScript1.2" src="modules/fckeditor/lib/fckeditor.js" type="text/javascript" ></script>';

This produce valid xhtml 1.0.