1.1 EPUB Revision History

EPUB had its roots in the interchange format known as the Open EBook Publication Structure (OEBPS). OEBPS 1.0 was approved in 1999 by the Open eBook Forum, an organization that later became the International Digital Publishing Forum (IDPF). Subsequent revisions 1.1 and 1.2 were approved by the IDPF in 2001 and 2002 respectively.

It was realized that a need existed for a format standard that could be used for delivery as well as interchange, and work began in late 2005 on a single-file container format for OEPBS, which was approved by the IDPF as the OEBPS Container Format (OCF) in 2006. Work on a 2.0 revision of OEBPS began in parallel which was approved as the renamed EPUB 2.0 in October, 2007, consisting of a triumvirate of specifications: Open Package Format (OPF), Open Publication Format (OPF) together with OCF. EPUB 2.0.1, a maintenance update to the 2.0 specification set primarily intended clarify and correct errata in the specifications, was approved in September, 2010. [OPF2] [OPS2] [OCF2]

Work on a major new 3.0 revision of the EPUB specifications began in 2010, with the goal of aligning EPUB more closely with HTML5, and in the process bringing new, native multimedia features, sophisticated CSS layout rendering and font embedding, scripted interactivity, enhanced global language support, and improved accessibility. A new specification, EPUB Media Overlays was also introduced, enabling text and audio synchronization in EPUB Publications. To better align the specification names with the standard, the Open Package Format specification was renamed EPUB Publications and the Open Publication Format specification was renamed EPUB Content Documents. The EPUB 3.0 specifications were approved in October, 2011. [Publications30] [ContentDocs30] [OCF30] [MediaOverlays30]

The EPUB 3.0.1 revision was undertaken in 2013-14. Although introducing mostly minor fixes and updates, it did see the integration of Fixed Layout Documents, which give Authors greater control over presentation when a reflowable EPUB is not suitable for the content. [Publications301] [ContentDocs301] [OCF301] [MediaOverlays301]

  3.1 Accessibility Support

EPUB 3.1 adds a recommendation that all EPUB Publications conform to the new EPUB Accessibility Specification [EPUBAccessibility]. The accessibility specification makes recommendations for the inclusion of discovery metadata along with [WCAG20] support in content.

EPUB Reading Systems are also recommended to meet the requirements in [EPUBAccessibility].

  3.2 New Core Media Types

EPUB 3.1 adds the WOFF 2.0 and application/font-sfnt font formats as Core Media Types.

  3.3 Clarification of Foreign Resource Fallbacks

EPUB 3.1 adds an additional clarification that foreign resources do not require fallbacks if they are not in the spine and not embedded in EPUB Content Documents.

This change will allow authors to include data files for use by scripts without unnecessary fallbacks, among other benefits.

  3.4 Removed support for EPUBCFI for linking

EPUB 3.1 removes the requirement for Reading Systems to support EPUB Canonical Fragment Identifiers [EPUBCFI] for hyperlinking (cf. the EPUB 3.0.1 EPUBCFI requirement).

  4.1 Dublin Core Metadata

The first Editor's Draft of EPUB 3.1 greatly reduced the available set of Dublin Core metadata elements. After considering additional feedback about the uses for the missing elements, all of the original elements have now been restored.

  4.2 The refines attribute

The refines attribute, previously used to chain metadata statements together, is superseded in EPUB 3.1. The following attributes have been added to replace the functionality it provided to Reading Systems: opf:scheme, opf:role, opf:alt-script, opf:file-as, opf:authority and opf:term.

  4.3 Linked Metadata Records

EPUB 3.1 also changes the precedence order of linked records, prioritizing bibliographic information in linked records over the information included directly in the Package Document metadata element. A protocol for the retrieval of metadata records hosted outside the container has also been defined.

  4.4 Removal of the OPF2 meta Element

EPUB 3.1 removes the [OPF2] meta element (cf. OPF2 meta element in EPUB 3.0.1).

  4.5 Removal of the NCX

The EPUB 2 NCX file for navigation is now marked as an obsolete feature (cf. EPUB 3.0.1 NCX). Support for it will be removed in the next major revision of EPUB.

  4.6 Removal of the guide Element

EPUB 3.1 removes the deprecated guide element (cf. EPUB 3.0.1 guide element).

  4.7 Removal of the bindings Element

EPUB 3.1 does not support the use of bindings in the Package Document to provide an alternative scripted fallback for foreign resources embedded in an object element (cf. EPUB 3.0.1 bindings).

The [HTML5] object element's intrinsic fallback mechanism (embedded content) remains for providing a Core Media Type fallback.

  5.1 Experimental Support for HTML Syntax Removed

The first Editor's Draft of EPUB 3.1 included support for the HTML syntax of HTML5. After considering community feedback from that draft, the EPUB Working Group decided to remove support. EPUB 3.1 will instead continue to support XHTML and SVG Content Documents, as was the case with EPUB 3.0.1.

Removal of the HTML syntax is not a reflection of a lack of desire for support in EPUB, but an acknowledgement that the EPUB 3 ecosystem is not yet stable and mature enough to handle the risks that would come with the introduction of a new type of Content Document. Issues with Reading System support and additional authoring complexity to support backwards compatibility were the primary concerns cited.

The EPUB Working Group sees support for the HTML syntax evolving two ways: 1) as part of the web-friendly format currently being developed as a separate specification in IDPF; and 2) through integration into the next major revision of EPUB (i.e., EPUB 4).

  5.2 Replacement of EPUB Style Sheets with CSS References

EPUB 3.1 removes the EPUB Style Sheets profile. In its place, more general CSS support requirements are being defined:

  5.3 Prioritization of Author and User Styles

EPUB 3.1 adds guidance that Reading Systems should prioritize the style choices of Authors and Users over its own user agent styles through the addition of Reading System conformance requirements and a subsection on overriding styles.

  5.4 Scripting Support Clarifications

EPUB 3.1 makes the following changes to scripting support:

  5.5 Removal of the switch Element

EPUB 3.1 does not support the inclusion of the switch element for conditional display of content (cf. EPUB 3.0.1 switch element).

Authors are instead directed to the existing guidance on the use of the MathML alttext and altimg attributes.

  5.6 Removal of the trigger Element

EPUB 3.1 does not support the inclusion of the trigger element for declarative control of audio and video content (cf. EPUB 3.0.1 trigger element).

Authors are advised to use the native controls provided by the [HTML5] audio and video elements.

  5.7 epubReadingSystem Object Clarifications

An IDL definition was added for the epubReadingSystem object.

The requirements for making the object available in the different scripting contexts, including in nested contexts, were also clarified.

  6.1 Encryption

The order of encryption and compression has been clarified. The new rules clarify that compression is not necessary when it is not beneficial to reducing the size of a resource. A new Compression element has been added the the encryption.xml schema to indicate whether a resource has been compressed and its original byte size.