Device-specific Graphics Compatibility

[Jaws]

I'm helping an author with a large back catalog from the 70s convert her out-of-print stuff to electronic formats, and I've run into a potential device-compatibility question that the collective mind here may be able to answer:

Are there compatibility problems, even for some older devices, with PNG graphics that have transparencies? And, especially, for devices in "dark mode" (consider a nicely-rendered calligraphic dropcap surrounded by a white square on a dark background...). Are those problems limited to certain file formats (.fb, for example, is a known issue)?