If you are building your ePUB3 book but wish it to be accessible offline readable for readers, you need to have these points in mind.
First, once a reader downloads a book, that book to read, it will become accessible offline.
However, ePUB3 allows for live links outs so that content from resources outside of a book are displayed.
So, here is what a publisher might have to consider in order to keep its book’s integrity.
1- Create alternatives which can be studied when the book is read offline.
2- Videos are important. For example you might have live links to Vimeo or YouTube. If you own the right or can have access to the file of the video, you can add that to the content of the book. Howeber, this may cause extra weight for the book which could occupy a large part of your reader’s computer or smart device.
We suggest that if you know that the book is going to be predominantly used in offline environment (for example textbooks for rural areas), then create shortened and concise version of videos and design based on internet accessibility beforehand.