From the International Digital Publishing Forum (IDPF):
The IDPF Membership has unanimously approved EPUB 3.1 as a Recommended Specification, by a vote of 72 YES to zero NO votes. 73 members voted (including one abstention) representing 54% of the members in good standing. significantly above the 25% quorum requirement.
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The EPUB 3.1 project began in October of 2015, with a goal of simplifying the specification and better aligning with the Open Web Platform. EPUB 3.1 is a significant update but includes no significant new features. it addresses many errata and other minor issues, reflects updates to HTML5 and other Web specifications since the finalization of EPUB 3.0.1 in June, 2014. and adds specificity to accessibility requirements. Additonally, the EPUB specification has been significantly reorganized, with general conformance requirements now a separate document that also serves as the main “entry point” to the EPUB specification.
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The EPUB 3.1 revision also introduces a new accessibility specification and a non-normative accessibility techniques document.
Learn More, Read the Complete Announcement