When an ebook has been created, it is important to ensure that it is ready for distribution and use by all readers. It is not enough to verify that it can be opened and viewed. It is also necessary to verify that the links all work, the images and tables are usable, the navigation is complete — in short, that all aspects of the ebook are usable by all readers. Is the ebook accessible — can all users, including those with various disabilities, make full use of the content? Ensuring that the ebook is fully ready for publication requires validating the ebook according to standards, such as the Web Content Accessibility Guidelines (WCAG 2.1) and EPUB Accessibility Standards (EPUB Accessibility 1.1). It also requires understanding the usability requirements of ebooks that go beyond these standards.
EPUB Preflight (https://epubpreflight.com — currently alpha) is our EPUB validator. It incorporates standard validation tools such as EPUBCheck and DAISY Ace. It also includes additional validations based on our own long experience successfully publishing ebooks that satisfy vendors and readers.