I used to work for a large US-based publisher with a big presence in education. I worked on the ePub parser and renderer written in React. As a company we basic

I used to work for a large US-based publisher with a big presence in education. I worked on the ePub parser and renderer written in React. As a company we basically took the standard and ran with it. Each book could have its own interactive widgets where kids could do reading comprehension questions or math problems and the system would capture all this for the teacher to grade. We had closed captioned audio for a lot books that the ePub reader would co-ordinate and play. Last I heard they abandoned all that for a completely proprietary format though. It's been the only situation where I've gotten elbow deep into implementing a specification. It was interesting feeling out the nuances and finding the optional parts of the spec that actually end up being important because it was all planned to fit together.