A change after a version number’s decimal point is usually minor or moderate, but the creators of the EPUB 3.1 draft at IDPF call their changes from 3.01 “radical.”
The EPUB working group has opted for a radical change approach to the addition and deletion of features in the 3.1 revision to move the standard aggressively forward with the overarching goals of alignment with the Open Web Platform and simplification of the core specifications.
It is expected that some of these radical changes will provoke strong reactions both for and against, as the working group has taken this approach to fully gauge the community response.
The big changes include requiring readers to support the HTML and XHTML syntax of HTML5 and replacing EPUB style sheets with broader CSS support. The old style sheets are a profile of CSS 2.1 with some bits of CSS3 added.
Maybe the real question at this point is whether there is — or should be — any difference between an EPUB document and a Web archive document. They’re certainly converging toward each other. The main requirement for an e-book, as opposed to a website, is that it can be read offline without external dependencies. A 2013 article by Adam Hyde sees this as a good thing, proposing that we should regard EPUB documents simply as “portable websites.” Maybe we should call the format “HTML/A,” by analogy to PDF/A and TI[FF]/A.
Mad File Format Science 