[Quoth]

Also many ebooks use pt, because Wordprocessors don't use em. All correctly rendering engines simply regard 12pt = 1em, thus everything is relative as intended. The purely Kobo proprietary kepub doesn't do this with pt, which is a bug. I've not figured out exactly what it was doing and kepub may treat pt on margins and font sizes differently, so I gave up and switched to epub for Kobo. All ebooks then have relative font sizes and margins as intended, no matter if pt or em.

However you have a choice. I now convert all ebooks added to Calibre anyway to automatically remove line-height and other stupidity. Sometimes I have to manually fix azw3, but only from the house of random penguins. So I have no problem with Standard eBooks epubs.

Calibre gives choice. It quick and trivial to have epub or automatically send kepub to a Kobo, so the same epub is sent as epub to Sony, Boyue, Apps, Y-ben, Binatone etc. I copy a load of PD ebooks to other family members. Not all have Kobos and some get azw3.