IDPF has released "EPUB 3.1 Now Proposed Specification."
Here's an excerpt:
Work on EPUB 3.1 began in October of 2015, with a goal of simplifying the format and better aligning with the Open Web Platform. . . .
The EPUB 3.1 revision also introduces a new accessibility specification and techniques document. Although developed as part of EPUB 3.1 and to provide guidance on making conforming EPUB publications accessible, these new documents are designed to be equally applicable to older versions of the specification.