Related or maybe not related, using superscript as a HTML element or as a span+css in text might cause problem with localization - in localization programs.
For example let's say "3rd party software":
3<sup>rd</sup> party software
3<span class="superscript">rd</span> party software
How can translators translate "rd"? They can leave it empty for several cyrrilic languages, but what about of other exotic or RTL languages?
In this case it is better to avoid using superscripts and use a full wording like "third-party software". Or, as mentioned here in other comments, adding plus signs in superscript via jQuery.