(Re)View Your ePUB2/3 Master File In Your Browser!
Whether professionally compiled or created on your own, always check your ebook master file visually. Triple validation (by 3 differing methods) of its code is not enough. Neither is loading that master file into your favorite device.
Each device has its limitations. You need an approach that remains free of marketplace / vendor bias and displays the ebook's fully designed attributes. Here is something that might help, and others will follow soon.
See “Resources > Internet” and scroll down to “Extensions“ near the bottom of that page.