https://web.archive.org/web/20240106090941/https://web.hypothes.is/blog/ebook-partnership/
Hypothes.is announced a partnership to bring annotation to epub e-books in 2017, but after 2018 no mention of it on their blog. Mailed them to ask about any progress since. Read someone's msg on Masto that they have trouble keeping epub annotations connected to the book (unlike e.g. my kindle annotations that reside in the book, but also are linked back to the location by my obsidian kindle plugin, maintaining the connection and reference. epub is xhtml so it should be doable.
epub is part of w3c standards (cause xhtml)