This is a very important issue. ePUB3 books are attractive because of many assets that they can have beyond their text. These could be quizzes, games, videos from say YouTube, a courseware from your LMS, etc. Some of these rely on access to internet.
We cannot use full potential of ePUB3 features without having access to internet.
So what does an author do. Should he or she assume that all readers have access to internet?
Answer depends on type of your book. Certainly some books can be designed with the assumption that its readers have full or near full access to internet at all times. Actually this is not an impossible assumption. Most cell carriers nowadays offer good and strong data links in most places.
But not all the time. What if you are in a flight which does not have WiFi access. Or what about educational equity if your book is designed as textbook. We know that even in rich countries, there are many families who might not afford internet in their homes.
Theoretically you can create your book for complete offline access and still use lots of ePUB3 features. This even means loading videos and interactive assets into the book.
However, all depends on acceptable size of the book. You do not want your book to be large to impact its download time and use of readers scarce smart device memory.
So there must be a compromise. Here are some tips
Try to add your interactive assets as HTML5 files into your book for offline access. KNiTO Interactive lets you do this.
Inside KNiTO Studio and when you are adding videos and graphics or links to outside resources try to use the "Modal" option which in effect lets you add an icon which needs to be clicked by the reader before an actual download starts. You can accompany that with some descriptive text and give the option to the reader not to click on the icon.
Reduce your video and other asset sizes. Perhaps by adding lower resolution alternatives for offline reads.
Add and use some of the guidances from accessibility in order to use of captions and similar options which will not only make the book effective in offline mode, they also let those with disabilities use the book efficiently.
There are lots of other things you can do in order to make offline reading experience satisfactory.
We welcome your comments and experiences so that we can add to the list here.