Our richtext doesn't support <emphasis> and <link> tags inside <superscript> and <subscript> tags.
Even though docbook is supposed to support that ( superscript, subscript), the docbook .rng schema do not accept it. Looks like a docbook bug.
EZP-28030, this makes it quite easy to create richtext doc that does not validate.
Also, it creates problems for ezxmltext -> richtext migration