One thing that people might find interesting is this eBook in the Library here -->
https://www.mobileread.com/forums/sho...d.php?t=211123
It's been used by people in the Kobo forums to look at items like font support / image rendering with different Kobo firmwares.
** EDIT
Bother; that link was not to the Font Tester I remember, and I'm currently not able to find the original post. I've attached the one I was thinking of that has images as well.