As an implementor on the Readium EPUB viewer, one thing that was noticeably absent in the spec was how to handle reader preferences.

For example these three things were configurable in the readium epub reader: font size, background color, and font color.

There is no one way to deterministically set these properties in every epub content document. This is because the spec does not address this scenario in spite of almost every epub reader allowing users to change these styling properties.

It's a matter of overriding the author's css which isn't incredibly difficult but there are many edge cases and it feels overall like a very hacky approach.