|
Have you opened the files in a hex editor to see what their magic (first four) bytes are?
It's unlikely that EPUB will become unreadable in the future, considering it's not a proprietary format; ZIP, HTML, CSS, and JPEG are all foundational elements of the web. Even if they get replaced by better formats in the future, there is such a wide variety of software that can read them you'd be hard pressed to find zero programs that work.
|