If you look at admin/jscript/tiny_mce/tiny_mce_setup.js.php, you can
look at line 55 or so and on for a list of valid elements. You could
make sure that the tag and attribute(s) you want to use are in that
list. There is also a way to switch modes for Tiny_MCE to/from (X)HTML
but you would have to search this out ... I am not sure where to
change this setting.