ISO/IEC TS 30135-1:2014
(Main)Information technology — Digital publishing — EPUB3 — Part 1: EPUB3 Overview
Information technology — Digital publishing — EPUB3 — Part 1: EPUB3 Overview
The EPUB® specification is a distribution and interchange format standard for digital publications and documents. EPUB defines a means of representing, packaging and encoding structured and semantically enhanced Web content — including HTML5, CSS, SVG, images, and other resources — for distribution in a single-file format. EPUB 3, the third major release of the standard, consists of a set of four specifications, each defining an important component of an overall EPUB Publication: -EPUB Publications 3.0 [Publications30], which defines publication-level semantics and overarching conformance requirements for EPUB Publications. -EPUB Content Documents 3.0 [ContentDocs30], which defines profiles of XHTML, SVG and CSS for use in the context of EPUB Publications. -EPUB Open Container Format (OCF) 3.0 [OCF3], which defines a file format and processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container. -EPUB Media Overlays 3.0 [MediaOverlays30], which defines a format and a processing model for synchronization of text and audio. EPUB has been widely adopted as the format for digital books (eBooks), and these new specifications significantly increase the format's capabilities in order to better support a wider range of publication requirements, including complex layouts, rich media and interactivity, and global typography features. The expectation is that EPUB 3 will be utilized for a broad range of content, including books, magazines and educational, professional and scientific publications. This document provides a starting point for content authors and software developers wishing to understand these specifications. It consists of non-normative overview material, including a roadmap to the four building-block specification documents that compose EPUB 3. Another non-normative document, EPUB 3 Changes from EPUB 2.0.1 [EPUB3Changes], describes changes in EPUB 3 from the previous version, but is intended primarily for Authors and EPUB Reading System vendors migrating from EPUB 2.0.1 to EPUB 3 and for those who anticipate supporting both versions.
Technologies de l'information — Publications numériques — EPUB3 — Partie 1: Aperçu général de EPUB3
General Information
Standards Content (Sample)
TECHNICAL ISO/IEC
SPECIFICATION TS
30135-1
First edition
2014-11-15
Information technology — Digital
publishing — EPUB3 —
Part 1:
EPUB3 Overview
Technologies de l'information — Publications numériques — EPUB3 —
Partie 1: Aperçu général de EPUB3
Reference number
ISO/IEC TS 30135-1:2014(E)
©
ISO/IEC 2014
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ISO/IEC TS 30135-1:2014(E)
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ISO/IEC TS 30135-1:2014(E)
Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are members of
ISO or IEC participate in the development of International Standards through technical committees
established by the respective organization to deal with particular fields of technical activity. ISO and IEC
technical committees collaborate in fields of mutual interest. Other international organizations, governmental
and non-governmental, in liaison with ISO and IEC, also take part in the work. In the field of information
technology, ISO and IEC have established a joint technical committee, ISO/IEC JTC 1.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of the joint technical committee is to prepare International Standards. Draft International
Standards adopted by the joint technical committee are circulated to national bodies for voting. Publication as
an International Standard requires approval by at least 75 % of the national bodies casting a vote.
In other circumstances, particularly when there is an urgent market requirement for such documents, the joint
technical committee may decide to publish an ISO/IEC Technical Specification (ISO/IEC TS), which
represents an agreement between the members of the joint technical committee and is accepted for
publication if it is approved by 2/3 of the members of the committee casting a vote.
An ISO/IEC TS is reviewed after three years in order to decide whether it will be confirmed for a further three
years, revised to become an International Standard, or withdrawn. If the ISO/IEC TS is confirmed, it is
reviewed again after a further three years, at which time it must either be transformed into an International
Standard or be withdrawn.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
ISO/IEC TS 30135 series were prepared by Korean Agency for Technology and Standards (as KS X 6070
series) with International Digital Publishing Forum and were adopted, under a special “fast-track procedure”,
by Joint Technical Committee ISO/IEC JTC 1, Information technology, in parallel with its approval by the
national bodies of ISO and IEC.
ISO/IEC TS 30135 consists of the following parts, under the general title Information technology — Document
description and processing languages — EPUB 3:
— Part 1: Overview
— Part 2: Publications
— Part 3: Content Documents
— Part 4: Open Container Format
— Part 5: Media Overlay
— Part 6: Canonical Fragment Identifier
— Part 7: Fixed-Layout Documents
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EPUB 3 Overview
Recommended Specification 11 October 2011
THIS VERSION
http://www.idpf.org/epub/30/spec/epub30-overview-20111011.html
LATEST VERSION
http://www.idpf.org/epub/30/spec/epub30-overview.html
PREVIOUS VERSION
http://www.idpf.org/epub/30/spec/epub30-overview-20110908.html
A diff of changes from the previous draft is available at this link.
Copyright © 2010, 2011 International Digital Publishing Forum™
All rights reserved. This work is protected under Title 17 of the United States Code. Reproduction and
dissemination of this work with changes is prohibited except with the written permission of the International
Digital Publishing Forum (IDPF).
EPUB is a registered trademark of the International Digital Publishing Forum.
Editors
Garth Conboy, Google Inc.
Matt Garrish, Invited Expert
Markus Gylling, DAISY Consortium
William McCoy, International Digital Publishing Forum (IDPF)
MURATA Makoto, JEPA EPUB Study Group
Daniel Weck, DAISY Consortium
TAB LE O F CO NTENTS
1. Introduction
1.1. Overview
1.2. Roadmap
2. Features
2.1. Package Document
2.2. Navigation
2.2.1. Reading Order
2.2.2. Navigation Document
2.3. Linking
2.4. Metadata
2.5. Content Documents
2.6. Rendering and CSS
2.7. Multimedia
2.8. Fonts
2.9. Scripting
2.10. Text-to-speech
2.11. Container
3. Global Language Support
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3.1. Metadata
3.2. Content Documents
3.3. CSS
3.4. Fonts
3.5. Text-to-speech
3.6. Container
4. Accessibility
4.1. Navigation
4.2. Semantic Markup
4.3. Dynamic Layouts
4.4. Aural Renditions and Media Overlays
4.5. Fallbacks
4.6. Scripting
A. Glossary
B. Acknowledgements and Contributors
B.1. EPUB 3.0
B.2. EPUB 2.0.1
B.3. EPUB 1.0
References
› 1 Introduction
› 1.1 Overview
The EPUB® specification is a distribution and interchange format standard for digital publications and
documents. EPUB defines a means of representing, packaging and encoding structured and
semantically enhanced Web content — including HTML5, CSS, SVG, images, and other resources — for
distribution in a single-file format.
EPUB 3, the third major release of the standard, consists of a set of four specifications, each defining an
important component of an overall EPUB Publication:
EPUB Publications 3.0 [Publications30], which defines publication-level semantics and
overarching conformance requirements for EPUB Publications.
EPUB Content Documents 3.0 [ContentDocs30], which defines profiles of XHTML, SVG and CSS
for use in the context of EPUB Publications.
EPUB Open Container Format (OCF) 3.0 [OCF3], which defines a file format and processing
model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container.
EPUB Media Overlays 3.0 [MediaOverlays30], which defines a format and a processing model for
synchronization of text and audio.
EPUB has been widely adopted as the format for digital books (eBooks), and these new specifications
significantly increase the format's capabilities in order to better support a wider range of publication
requirements, including complex layouts, rich media and interactivity, and global typography features.
The expectation is that EPUB 3 will be utilized for a broad range of content, including books, magazines
and educational, professional and scientific publications.
This document provides a starting point for content authors and software developers wishing to
understand these specifications. It consists of non-normative overview material, including a roadmap to
the four building-block specification documents that compose EPUB 3.
Another non-normative document, EPUB 3 Changes from EPUB 2.0.1 [EPUB3Changes], describes
changes in EPUB 3 from the previous version, but is intended primarily for Authors and EPUB Reading
System vendors migrating from EPUB 2.0.1 to EPUB 3 and for those who anticipate supporting both
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versions.
› 1.2 Roadmap
This section provides an overview of the EPUB 3 specifications by explaining in brief the components of a
Publication. Links to additional information within this document and to the specifications are included.
An EPUB Publication, at its most basic level, is a bundled collection of resources that can be reliably
and predictably ingested by an EPUB Reading System in order to render its contents to a User. Some of
these resources facilitate the discovery and processing of the EPUB Publication, while others make up
the content of the source publication. The latter, EPUB Content Documents, are described in Content
Documents and are fully defined in [ContentDocs30].
A Publication's resources are typically bundled for distribution as a ZIP-based archive with the file
extension .epub. As conformant ZIP archives, Publications can be unzipped by many software programs,
simplifying both their production and consumption. The container format is introduced in Container and
defined in [OCF3].
The container format not only provides a means of determining that the zipped content represents an
EPUB Publication (the mimetype file), but also provides a universally-named directory of informative
resources (/META-INF). Key among these is the container.xml file, which directs Reading Systems to the
root file of the Publication, the Package Document.
The Package Document is itself a kind of information warehouse for the Publication, storing metadata
about the specific work contained in the Publication, providing an exhaustive list of resources and defining
a default reading order. The Package Document is introduced in Package Document and defined in
[Publications30].
The preceding components of an EPUB Publication are not new to EPUB 3, and will be familiar to
anyone who has worked with Publications before, although they have been changed and enhanced in this
version. A new core addition to EPUB 3, however, is the Media Overlay Document, which defines a
means of synchronizing text and audio playback. The Overlay Document is introduced in Multimedia and
defined in [MediaOverlays30].
The following example shows the resources a minimal "Hello World" Publication might contain:
mimetype
META-INF/container.xml
Content/HelloWorld.opf
Content/HelloWorld.xhtml
While conceptually simple, an EPUB Publication is more than just a collection of HTML pages and
dependent assets in a ZIP package as represented in this example. The following sections of this
document delve into more detail about the primary features and functionality that Publications provide to
enhance the reading experience.
› 2 Features
This section covers the major features of EPUB, including important components and topics that apply to
the process of authoring EPUB Publications as a whole.
2.1 Package Document
›
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Every EPUB Publication includes a single Package Document, which specifies all the Publication's
constituent content documents and their required resources, defines a reading order for linear
consumption, and associates Publication-level metadata and navigation information.
The Package Document represents a significant improvement on a typical Web site. A Web site, for
example, embeds references to its resources within its content, which, while a simple and flexible means
of identifying resources, makes it difficult to enumerate all the resources required to render it. In addition,
there is no standard way for a Web site to define that a sequence of pages make up a larger publication,
which is precisely what EPUB's spine element does (i.e., it provides an external declarative means to
explicitly specify navigation through a collection of documents). Finally, the Package Document defines a
standard way to represent metadata globally applicable to a collection of pages.
The Package Document and other Publication-level constructs are specified in [Publications30].
› 2.2 Navigation
› 2.2.1 Reading O rder
A key concept of EPUB is that a Publication consists of multiple resources that may be completely
navigated and consumed by a person or program in some specific order.
Many publications have an obvious reading order, or logical progression through their content. A novel is
an example of a highly sequential document — it typically has a beginning, middle and end — but not all
publications are so ordered: a cookbook or collection of photographic images might be considered to be
more like a database. All documents do, however, have at least one logical ordering of all their top-level
content items, whether by date, topic, location or some other criteria (e.g., a cookbook is typically
ordered by type of recipe).
Every EPUB Publication defines at least one such logical ordering of all its top-level content (the spine
[Publications30]), as well as a declarative table of contents (the EPUB Navigation Document
[ContentDocs30]). Publications make these data structures available in a machine-readable way external
to the content, simplifying their discovery and use.
EPUB Publications are not limited to the linear ordering of their contents, nor do they preclude linking in
arbitrary ways — just like the Web, EPUB Publications are built on hypertext — but the basic
consumption and navigation can be reliably accomplished in a way that is not true for a set of HTML
pages.
› 2.2.2 Navigation Document
Every EPUB Publication contains a special XHTML Content Document called the EPUB Navigation
Document, which uses the HTML5 nav element to define human- and machine-readable navigation
information.
The Navigation Document supersedes the NCX document [OPS2], and the inclusion of NCX documents
is only recommended for forward compatibility in older Reading Systems. The Navigation Document,
while maintaining the baseline accessibility and navigation support and features of the NCX, introduces
new functionality and rendering features to enhance navigation for all Users. Prime among these are
better support for internationalization (as an XHTML5 document itself, the Navigation Document natively
supports ruby annotations) and support for embedded grammars (MathML and SVG can be included
within navigation links).
As XHTML Content Documents, Navigation Documents also provide a flexible means of tailoring the
navigation display using CSS and the hidden attribute [ContentDocs30] while not impacting access to
information for accessible Reading Systems.
The structure and semantics of Navigation Documents are defined in EPUB Navigation Documents
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[ContentDocs30].
› 2.3 Linking
The new EPUB Canonical Fragment Identifier (epubcfi) Specification [EPUBCFI] defines a standardized
method for linking into a Publication.
Required support for this scheme in Reading Systems means that EPUB now has an interoperable
linking mechanism, one that can, for example, facilitate the sharing of bookmarks and reading locations
across devices.
› 2.4 Metadata
EPUB Publications provide a rich array of options for adding Publication metadata. The Package
Document includes a dedicated metadata section [Publications30] for general information about the
Publication, allowing titles, authors, identifiers and other information about the Publication to be easily
accessed. It also provides the means to attach complete bibliographic records to a Publication using the
link element [Publications30].
The Package Document also allows a Unique Identifier to be established for a Publication using the
unique-identifier attribute [Publications30]. The required last-modified date in the Package metadata
section can be joined with this identifier to define a Package Identifier, which provides a means of
distinguishing EPUB Publications that represent different versions of the same Manifestation (see
Publication Identifiers [Publications30]). The Package Identifier addresses the issue of how to release a
Publication without changing its Unique Identifier while still identifying it as a new version.
XHTML Content Documents also include the means of annotating document markup with rich metadata,
making them more semantically meaningful and useful both for processing and accessibility purposes
(Semantic Inflection [ContentDocs30]).
› 2.5 Content Documents
Every EPUB Publication contains one or more EPUB Content Documents, as defined in
[ContentDocs30]. These are XHTML or SVG documents that describe the readable content of a
Publication and reference associated media resources (e.g., images, audio and video clips).
XHTML Content Documents are defined by a profile of HTML5 that requires the use of XML serialization
[HTML5] in order to ensure that content can be reliably manipulated and rendered. This profile also adds
two additional EPUB-specific language constructs: the epub:type attribute [ContentDocs30] for element-
level metadata and the epub:trigger element [ContentDocs30] for declaratively associating controls with
multimedia elements.
These additions do not affect the ability of an HTML5 User Agent [HTML5] to render EPUB XHTML
Content Documents, but Publications might not render identically in all User Agents depending on their
support.
› 2.6 Rendering and CSS
A key concept of EPUB is that content presentation should adapt to the User rather than the User having
to adapt to a particular presentation of content. HTML was originally designed to support dynamic
rendering of structured content, but over time HTML as supported in Web browsers has become focused
on the needs of Web applications, and most popular Web sites now have fixed-format layouts.
EPUB Publications, however, are designed to maximize accessibility for the visually impaired, and
Reading Systems typically perform text line layout and pagination on the fly, adapting to the size of the
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display area, the User's preferred font size, and other environmental factors. This behavior is not
guaranteed in EPUB; images, vector graphics, video and other non-reflowable content may be included,
and some Reading Systems might not paginate on the fly, or at all. Nevertheless, supporting dynamic
adaptive layout and accessibility has been a primary design consideration throughout the evolution of the
EPUB standard.
EPUB Content Documents may optionally reference EPUB Style Sheets, allowing Authors to define the
desired rendering properties. EPUB 3 defines a profile of CSS based on CSS 2.1 [CSS2.1] for this
purpose, together with capabilities defined by various CSS3 Modules and several additional properties
specific to EPUB.
CSS3 properties were selected based on their current level of support in Web browsers, but support for
them in Reading Systems and User Agents is not guaranteed (EPUB-defined properties may similarly be
ignored).
EPUB 3 also supports CSS styles that enable both horizontal and vertical layout and both left-to-right
and right-to-left writing, but Reading Systems might not support all of these capabilities. Reading
Systems may also support different rendering options than the Author intended. Refer to CSS in the
Global Language Support section for more information.
EPUB 3 also supports the ability to include multiple style sheets that allow users, for example, to select
between day/night reading modes or to change the rendering direction of the text. Refer to Alternate Style
Tags [ContentDocs30] for more information.
› 2.7 Multimedia
EPUB 3 supports audio and video embedded in [content documents] via the new [HTML5] audio and
video elements, inheriting all the functionality and features these elements provide. (For information on
supported audio formats, please refer to Core Media Types [Publications30]. For recommendations on
embedding video, refer to Reading System Conformance [Publications30].)
Another key new multimedia feature in EPUB 3 is the inclusion of Media Overlay Documents
[MediaOverlays30]. When pre-recorded narration is available for a Publication, Media Overlays provide the
ability to synchronize that audio with the text of a Content Document (see also Aural Renditions and
Media Overlays).
2.8 Fonts
›
EPUB 3 supports two closely-related font formats — OpenType [OpenType] and WOFF [WOFF] — to
accommodate both traditional publishing workflows and emerging Web-based workflows. Word
processing programs used to create Publications are likely to have access only to a collection of
installed OpenType fonts, for example, whereas Web-archival EPUB generators will likely only have
access to WOFF resources (which cannot be converted to OpenType without undesirable, and potentially
unlicensed, stripping of WOFF metadata).
EPUB 3 also supports both obfuscated and regular font resources for both OpenType and WOFF font
formats. Support for obfuscated font resources is required to accommodate font licensing restrictions for
many commercially-available fonts.
› 2.9 Scripting
EPUB strives to treat content declaratively — as data that can be manipulated, not programs that must
be executed — but does support scripting as defined in HTML5 and SVG (refer to Scripted Content
Documents [ContentDocs30] for more information).
It is important to note, however, that scripting support is optional for Reading Systems and may be
disabled for security reasons.
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Authors should also note that scripting in an EPUB Publication can create security considerations that
are different from scripting within a Web browser. For example, typical same-origin policies are not
applicable to content that has been downloaded to a User's local system. Therefore, it is strongly
encouraged that scripting be limited to container constrained contexts, as further described in Scripted
Content Documents — Content Conformance [ContentDocs30].
Scripting consequently should be used only when essential to the User experience, since it greatly
increases the likelihood that content will not be portable across all Reading Systems and creates barriers
to accessibility and content reusability.
› 2.10 Text-to-speech
EPUB 3 provides the following text-to-speech (TTS) facilities for controlling aspects of speech synthesis,
such as pronunciation, prosody and voice characteristics:
Pronunciation Lexicons
The inclusion of generic pronunciation lexicons using the W3C PLS format [PLS] enables Authors
to provide pronunciation rules that apply to the entire EPUB Publication. Refer to PLS Documents
[ContentDocs30] for more information.
Inline SSML Phonemes
The incorporation of SSML phonemes functionality [SSML] directly into a EPUB Content
Document [ContentDocs30] enables fine-grained pronunciation control, taking precedence over
default pronunciation rules and/or referenced pronunciation lexicons (as provided by the PLS
format mentioned above). Refer to SSML Attributes [ContentDocs30] for more information.
CSS Speech Features
The inclusion of a select set of features from the CSS 3 Speech Module [CSS3Speech]
(previously known as CSS 2.1 Aural Stylesheets [CSS2.1]) enables Authors to control further
speech synthesis characteristics. Refer to CSS 3.0 Speech [ContentDocs30] for more
information.
› 2.11 Container
An EPUB Publication is transported and interchanged as a single file (a "portable document") that
contains the Package Document, all Content Documents and all other required resources for processing
the Publication. The single-file container format for EPUB is based on the widely adopted ZIP format. An
XML manifest that specifies the location in the ZIP archive of the Package Document must be found at a
well-defined location within the archive.
This approach provides a clear contract between any creator of an EPUB Publication and any system
which consumes such Publications, as well as a reliable representation that is independent of network
transport or file system specifics.
An EPUB Publication's representation as a container file is specified in [OCF3].
› 3 Global Language Support
› 3.1 Metadata
EPUB 3 supports alternate representations of all text metadata items in the package metadata section to
improve global distribution of Publications. The alternate-script property can be combined with the
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xml:lang attribute to include and identify alternate script renditions of language-specific metadata.
Using this property, a Japanese Publication could, for example, include an alternate Roman-script
representation of the author's name and/or one or more representations of the title in Romance
languages. Refer to the alternate-script property [Publications30] for more information.
The page-progression-direction attribute also allows the content flow direction to be globally specified for
all Content Documents to facilitate rendering (see the page-progression-direction [Publications30]).
› 3.2 Content Documents
XHTML Content Documents leverage the new HTML5 directionality features to improve support for
bidirectional content rendering: the bdi element allows an instance of directional text to be isolated from
the surrounding content, the bdo element allows directionality to be overridden for its child content and the
dir attribute allows the directionality of any element to be explicitly set.
XHTML Content Documents also support ruby annotations for pronunciation support (which makes them
supported in Navigation Document links, as well).
SVG Content Documents support the rendering of bidirectional text, but do not include support for ruby.
› 3.3 CSS
EPUB 3's support for new CSS3 modules enables typography for many different languages and cultures.
Some specific enhancements include:
support for vertical writing, which also provides Reading Systems the ability to allow users to
toggle direction;
better handling of emphasis, such as the inclusion of bōten;
better control over line breaking, so that breaks can occur at the character level for languages that
do not use spaces to delimit new words; and
better control over hyphenation, to further facilitate line breaking.
› 3.4 Fonts
EPUB 3 does not require that Reading Systems come with any particular set of built-in system fonts. As
occurs in Web contexts, Users in a particular locale may have installed fonts that omit characters
required for other locales, and Reading Systems may utilize intrinsic fonts or font engines that do not
utilize operating system installed fonts. As a result, the text content of a Publication might not natively
render as intended on all Reading Systems.
To address this problem, EPUB 3 supports the embedding of fonts to facilitate the rendering of text
content, and this practice is recommended in order to ensure content is rendered as intended.
Support for embedded fonts also ensures that Publication-specific characters and glyphs can be
embedded for proper display.
3.5 Text-to-speech
›
EPUB 3's support for PLS documents and SSML attributes increases the pronunciation control that
Authors have over the rendering of any natural language in text-to-speech-enabled Reading Systems.
Refer to Text-to-speech in the Features section for more information on these capabilities.
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The combination of CSS Speech and inline SSML phonemes also allows fine control over ruby.
› 3.6 Container
The OCF container format supports UTF-8, allowing for internationalized file and directory naming of
content resources.
› 4 Accessibility
A major goal of EPUB is to facilitate content accessibility, and a variety of features in EPUB 3 support
this requirement. This section reviews these features, detailing some established best practices for
ensuring that EPUB Publications are accessible where applicable.
It is important to note that while accessibility is important in its own right, accessible content is also
more valuable content: an accessible Publication will be adaptable to more devices and be easier to
reuse, in whole or in part, via human and automated workflows. The EPUB Working Group strongly
recommends that Authors use EPUB tools that generate accessible content.
› 4.1 Navigation
EPUB 3 improves on NCX documents with the addition of EPUB Navigation Documents. As noted in
Content Documents above, the new features these provide represent a more universal and flexible
navigation system.
The need to navigate a document is not exclusively an accessibility issue, but a complete Navigation
Document is key for all Users to be able to easily and fully access the contents of a document. The
primary toc nav element (as defined in The toc nav Element [ContentDocs30]) should fully reflect the
structure of the Publication, as a result.
For highly structured documents where it might not be desirable to display the complete table of contents
to Users, the display level can be modified using the [HTML5] hidden attribute without minimizing the
information that is available for accessible Reading Systems.
Authors are also encouraged to supply additional nav elements if their Publications contain non-structural
points of interest, such as figures, tables, etc. in order to further enhance access to the content.
4.2 Semantic Markup
›
HTML5 supports a number of new elements intended to make markup more semantically meaningful
(e.g., section, nav, aside) and introduces more clearly defined semantics for some HTML4 elements.
These elements, in conjunction with best practices for authoring well-structured Web content, should be
utilized when creating EPUB XHTML Content Documents. These additions allow content to be better
grouped and defined, both for representing the structure of documents and to facilitate their logical
navigation. XHTML Content Documents also natively support the inclusion of ARIA role and state
attributes and events, enhancing the ability of Assistive Technologies to interact with the content.
EPUB 3 further introduces the epub:type [ContentDocs30] attribute, which is meant to be functionally
equivalent to the W3C Role Attribute [Role].
...
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