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The future of EPUB? A first look at the EPUB 3.1 Editor’s draft


About a month ago the International Digital Publishing Forum, the standards body behind the EPUB format, published an Editor’s Draft of EPUB 3.1. This is meant to be the successor of the current 3.0.1 version. IDPC has set up a community review, which allows interested parties to comment on the draft. The proposed changes relative to EPUB 3.0.1 are summarised in this document. A note at the top states (emphasis added by me):

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.

As Gary McGath pointed out earlier, this is a pretty bold statement for what is essentially a minor version. The authors of the draft also mention that they expect it “will provoke strong reactions both for and against”, and that changes that raise “strong negative reactions” from the community “will be reviewed for future drafts”.

This blog post is an attempt to identify the main implications of the current draft for libraries and archives: to what degree would the proposed changes affect (long-term) accessibility? Since the current draft is particularly notable for its aggressive removal of various existing EPUB features, I will focus on these. These observations are all based on the 30 January 2016 draft of the changes document.

Removed support for EPUBCFI for linking

The EPUB Canonical Fragment Identifier (EPUBCFI) “defines a standardized method for referencing arbitrary content within an EPUB Publication”. Until EPUB 3.0.1, Reading Systems were required to support EPUBCFI for hyperlinking within and between documents. This requirement is dropped in EPUB 3.1 (although it would still be possible to use EPUBCFI for annotations and bookmarks).

In principle this change could result in problems if an EPUB that uses CFI for hyperlinks is opened in a 3.1 reading system: in that case the hyperlinks would not work. However, according to EPUB editor Matt Garrish, authors simply do not use CFI for hyperlinking. He also mentions a check by Google on their corpus of millions of books, which only turned up a few instances of CFI use. One of these was a link in an EPUB best practices book, while the remaining ones were all part of the EPUB test suite documents. If these results are representative of all EPUBs “in the wild”, the implications of the change would be negligible.

Reduced set of metadata elements in Package Document

EPUB 3.1 imposes restrictions on the metadata elements that can be embedded in the Package Document. Up to version 3.0.1, the full Dublin Core Metadata Element Set was supported, whereas in 3.1 only the dc:identifier, dc:title, dc:language, dc:creator, dc:publisher and dc:type elements are allowed. Additional metadata can be included, but they need to be defined in a separate resource (file), which is referenced from the metadata element using the link element. Below is an example that uses a MARC file:

<link rel="record"

Complicating things further, the EPUB 3.1 Packages draft says:

Linked resources that are not Publication Resources are not subject to Core Media Type requirements [EPUB31] and may be located inside or outside [EPUB31] the EPUB Container. Retrieval of Remote Resources is optional.

So, linked metadata resources can have any possible format, and they may not even be included in the EPUB container. Even though these changes would have no direct consequences for long-term accessibility, they would seriously complicate document processing (e.g. ingest) workflows that rely on the metadata in the Package Document. It would also affect end users who rely on these metadata fields to sort and find their ebooks.

Note: the discussion thread on this topic in the issue tracker is worth checking out, as it contains some excellent additional observations.

Removal of the NCX

EPUB 2 documents contain the NCX file (“Navigation Control file for XML”), which provides a mechanism to navigate a publication. It is essentially a hierarchical table of contents. The NCX was superseded by the Navigation Document in EPUB 3.0.1. However, the NCX was allowed in EPUB 3.01 publications, which was useful for keeping EPUB 3 publications compatible with older (EPUB 2-based) reading systems1. The 3.1 draft forbids the NCX altogether, which means that such “hybrid” EPUBs are not possible without breaking the specification.

The main consequence of this is that it would make EPUB 3.1 files incompatible with older reading systems. More specifically, basic navigation functionality such as direct access to a chapter from the table of contents would not work.

To get an approximate idea of the impact of this, I had a look at the EPUB 3 support grid, which gives detailed information about the support of specific EPUB 3 features for commonly used devices, apps, and reading systems. This link shows support of the toc nav element, which defines the primary navigational hierarchy in the Navigation Document. Only 55% (34 out of 62) of all tested reading systems fully support the toc nav element, with 37% (23 out of 62) not supporting it at all2. This may not be a big deal for users of software-based reading systems (which make up the majority of the support grid), but users of (older) E-ink readers often don’t have the option to upgrade their devices. A good example is this (now discontinued) Sony e-Ink hardware reader. Unfortunately, E-ink devices appear to be underrepresented in the support grid. For example, it contains no information whatsoever on any of the popular Kobo readers.

The proposal to remove the NCX provoked strong reactions in the community review, with one respondent stating it would lead to “dropping support for millions of eInk reading systems”. It would also contradict this statement from the EPUB 3.0.1 specification (emphasis added by me):

The NCX feature defined in [OPF2] is superseded by the EPUB Navigation Document [ContentDocs301]. EPUB 3 Publications may include an NCX (as defined in OPF 2.0.1) for EPUB 2 Reading System forwards compatibility purposes, but EPUB 3 Reading Systems must ignore the NCX.

The explicit reference to EPUB 3 Publications (not EPUB 3.0.1 Publications!!) implies that the statement applies to EPUB 3 in general. Removing the NCX in another EPUB 3 release would be at odds with this.

Removal of the guide Element

The guide element was an optional data structure in EPUB 2 that provided “convenient access” to structural components of a publication. It was deprecated in EPUB 3.0.1. Without any data on the actual usage of this feature, it is difficult to say much about the impact of its complete removal (this was also pointed out by one respondent to the community review).

Removal of the bindings Element

In EPUB 3.0.1 the bindings element could be used to define fallbacks for foreign resources. According to EPUB editor Matt Garrish “this feature is not widely used or supported”, and the impact on accessibility appears to be negligible.

Removal of the switch Element

The switch element in EPUB 3.0.1 allows one to define alternative representations of XML fragments. Here’s an example:

<epub:switch id="cmlSwitch">
   <epub:case required-namespace="http://www.xml-cml.org/schema">
      <cml xmlns="http://www.xml-cml.org/schema">
         <molecule id="sulfuric-acid">
            <formula id="f1" concise="H 2 S 1 O 4"/>

Here, we have a chemical formula in ChemML format and in standard HTML. ChemML is not natively supported in EPUB, so by default a reader will display the HTML version. However, wrapping both in a switch element would allow a ChemML-capable reader to render that representation instead.

I asked EPUB editor Matt Garrish how an EPUB 3.1-compliant reader would render content that is wrapped in a switch element. He replied that by default all of the switch content would be rendered. So for the example above, a reader would try to render both the HTML and the ChemML versions (with the latter failing on most reading systems). Matt stressed the significance of the switch element, adding that people have been using it, “if not extensively”.

Removal of the trigger Element

The trigger element in EPUB 3.0.1 is used to define simple user interfaces for multimedia content. Since this can be done natively in HTML 5, it is dropped from EPUB 3.1. Here editor Matt Garrish explains that the feature is both “sparsely used” (referring to a survey of publishers) and “poorly supported”.

Miscellaneous changes

Apart from the changes above (which all remove features from the existing specification), the EPUB 3.1 draft also adds a number of new features, and clarifies some existing ones. I won’t go over them in detail, but here’s a brief overview:

Finally, the draft contains clarifications on Foreign Resource Fallbacks and Scripting Support.

EPUB 3.1 or EPUB 4.0?

By now it should be clear that the aggressive removal of features in EPUB 3.1 would have some far-reaching consequences. This is particularly true for the removal of the NCX, which would make EPUB 3.1 files incompatible with many existing E-ink readers. It would do this by ruling out the option to make backward-compatible “hybrid” files. As Gary McGath pointed out earlier, introducing “radical changes” in what is essentially a minor version is pretty unusual practice for any standard. Nowadays, most software and file formats use some variation of semantic versioning, with version numbers that follow the general form MAJOR.MINOR.PATCH. Here, each component of the version number has a well-defined meaning:

  1. MAJOR version is increased in case of incompatible API changes,
  2. MINOR version is increased when functionality is added in a backwards-compatible manner, and
  3. PATCH version is increased in case of backwards-compatible bug fixes.

Since the current draft includes multiple backward-incompatible changes, this makes me wonder why the editors didn’t name it EPUB 4.0 instead! Kovid Goyal, lead developer of the popular Calibre software, made the following comment on this:

[I]f you want to make backwards incompatible changes, please, dont do it in a point release. From glancing over your changes document, it seems to me that you want to make several breaking changes. That’s great, EPUB 3 could do with some serious breaking. But name it EPUB 4. I really dont want to have tell my users that calibre supports EPUB 3.1 but not EPUB 3.

I agree with Kovid here. Having multiple sub-versions of EPUB 3, with some of them being backward-compatible with EPUB 2, while this backward compatibility is explicitly ruled out in another sub-version, is bound to create a situation that will be incomprehensible for most e-book buyers. Worse, it could even undermine overall confidence in the format. For memory institutions it would also make the management of EPUB 3 publications unnecessarily complicated. Not only would some EPUB 3.1 files not render correctly in an EPUB 3.0.1 reader, the opposite would be true as well.


In my 2012 report on EPUB for archival preservation I already mentioned the stability of the EPUB format as a concern:

EPUB 3 shows quite major changes relative to version 2, which raises concerns about the format’s stability over time. These concerns are reinforced by the fact that EPUB 3 is heavily dependent on (X)HTML5 and CSS3, both of which are unfinished “works in progress”, which may undergo various changes before being finalised.

These concerns are once more confirmed by the current EPUB 3.1 draft. However, it remains to be seen how many of these changes will make it to the final version. The community review process is ongoing at this moment, so if you’re getting a little uneasy after reading this blog post, there’s still time to get involved and make your voice heard!


Thanks to Matt Garrish for his prompt replies to my questions on Github.

  1. See here how O’Reilly’s keeps their EPUB 3 books compatible with EPUB 2 readers
  2. This figure includes reading systems for which support is unknown
  3. See the HTML5 Reference for a discussion of the differences between both syntaxes



  1. I’d just note that you don’t have to take my word on support in reading systems. If you go to epubtest.org, you can check support for a variety of features. You’ll find a lot of red in the fallback features, for example: http://epubtest.org/testsuite/epub3/feature/fallbacks/

    And the aggressiveness for the first draft was to gauge what is actually being used, not to impose the removals. The feedback received has been very helpful, and a variety of changes are in the works. Reviewing the NCX removal is on tomorrow’s agenda.

  2. Johan van der Knijff

    March 11, 2016 at 1:33 pm

    Thanks for getting back to this Matt. It didn’t escape my attention that the draft is intentionally provocative in places, and I agree this is an excellent way to poke people for feedback. I also like the use of Github issues for the review process, this makes everything really transparent while at the same time keeping the barriers for participating low. In an earlier draft of my blog post I actually mentioned these things in the final section, but that part of the blog didn’t make the final cut (I originally aimed for a 1000 word piece and went way over that!). The NCX sticks out as one of the biggies to me, so I’m curious about the outcome of that.

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