By adding a `<sup class="attribution-link">` I'm betting the HTML looks like this: `<sup><a href="..."><sup class="attribution-link">*</sup></a></sup>`

Check https://github.com/Connexions/oer.exports/blob/master/xsl/dbkplus.xsl#L421 to see how `<sup>` tags are currently generated. I'd suggest trying to use that template 1st.