To make sure that files are accepted by all vendors we distribute to, ePubs need to validate to EpubCheck v. 3.0.1, which some vendors still use to validate files.
There are errors that appear when validating to 3.0.1 that don’t appear when validating to 3.0 or 4.0.1. For example, a common problem that prevents validation is the stylesheet being a template formatted for Adobe Digital Editions (ADE). Since the major sites and devices do not run Digital Editions, it is not necessary to format the ePub specifically for display in that program. If you are unable to provide files that validate, we work with a vendor that can convert your files for you.