Whitelist sup tag for rich text
The sup tag does not work in rich text mode, but sub does. I noticed this is an issue with both Pleroma and Glitch-soc, discovered when I fixed it in my Mastodon fork that bases its rich text off of Glitch's implementation.
I am not sure if this will be an issue with Pleroma FE, but it might with Glitch FE; by default there is no styling on the sup tag and it needs to be added as follows to display properly:
sup {
font-size:xx-small;
vertical-align:super;
}