> plain old epubs

Kobo devices ship two different rendering engines: the obsolete, appallingly bad Adobe Reader Mobile SDK (RMSDK), and their own, bespoke, WebKit-based renderer. By default, files with an .epub extension are rendered with RMSDK. Consequently, you'll get all sorts of weird issues. Some of the ones I've encountered: I've had cover images not displaying in the library, misbehaving internal links (footnotes), inability to override typefaces/text size.

Easy fix: rename the file to .kepub.epub[0].

Slightly more involved fix: run the file through Kepubify[1] first. This tool makes further tweaks to the books to enable a few other features, like remaining read time estimation.

[0]: https://github.com/kobolabs/epub-spec#sideloading-for-testin...

[1]: https://pgaskin.net/kepubify/

Interesting, I wasn't aware of this, but it's good to know. My needs haven't been demanding enough to notice a difference.

Calibre can also do that for you transparently when loading the files onto the Kobo.