I’d like to format footnotes, is there a standard CSS class available?
I’m not aware of anything along those lines, as a standard say, but there are some interesting approaches out there, like this one with an emphasis on accessibility.
Granted that is not just CSS being demonstrated there, but I think it is worthwhile to note that there probably is no such thing as a pure-CSS footnote since footnotes don’t exist as a dedicated feature—rather they are something we make using smaller generic tools. How they are created is up to the creator. Even with Scrivener you will find different HTML structures depending on the file format you use.