ISO/IEC 23736-1:2020
(Main)Information technology — Digital publishing — EPUB 3.0.1 — Part 1: Overview
Information technology — Digital publishing — EPUB 3.0.1 — Part 1: Overview
This specification, EPUB Media Overlays 3.0.1, defines a usage of [SMIL] (Synchronized Multimedia Integration Language), the Package Document, the EPUB® Style Sheet, and the EPUB Content Document for representation of audio synchronized with the EPUB Content Document. This specification is one of a family of related specifications that compose EPUB 3, the third major revision of an interchange and delivery format for digital publications based on XML and Web Standards. It is meant to be read and understood in concert with the other specifications that make up EPUB 3: The EPUB 3 Overview [EPUB3Overview], which provides an informative overview of EPUB and a roadmap to the rest of the EPUB 3 documents. The Overview should be read first. EPUB Publications 3.0.1 [Publications301], which defines the semantics and overarching conformance requirements for each Rendition of an EPUB Publication. EPUB Content Documents 3.0.1 [ContentDocs301], which defines profiles of XHTML, SVG and CSS for use in the context of EPUB Publications. EPUB Open Container Format (OCF) 3.0.1 [OCF301], which defines a file format and processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container.
Technologies de l'information — Publications numériques — EPUB 3.0.1 — Partie 1: Aperçu général
General Information
Standards Content (Sample)
INTERNATIONAL ISO/IEC
STANDARD 23736-1
First edition
2020-02
Information technology — Digital
publishing — EPUB 3.0.1 —
Part 1:
Overview
Technologies de l'information — Publications numériques — EPUB
3.0.1 —
Partie 1: Aperçu général
Reference number
ISO/IEC 23736-1:2020(E)
©
ISO/IEC 2020
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ISO/IEC 23736-1:2020(E)
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ISO/IEC 23736-1:2020(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.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of document should be noted (see www.iso.org/directives).
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.
Details of any patent rights identified during the development of the document will be in the
Introduction and/or on the ISO list of patent declarations received (see www.iso.org/patents) or the IEC
list of patent declarations received (see http://patents.iec.ch).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT),
see www.iso.org/iso/foreword.html.
This document was prepared by the World Wide Web Consortium (W3C) (as EPUB 3 Overview) and
drafted in accordance with its editorial rules. It was adopted, under the JTC 1 PAS procedure, by Joint
Technical Committee ISO/IEC JTC 1, Information technology.
A list of all parts in the ISO/IEC 23736 series can be found on the ISO websitte.e
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
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ISO/IEC 23736-1:2020(E)
EPUB 3 Overview
Recommended Specification 26 June 2014
THIS VERSION
http://www.idpf.org/epub/301/spec/epub-overview-20140626.html
LATEST VERSION
http://www.idpf.org/epub3/latest/overview
PREVIOUS VERSION
http://www.idpf.org/epub/301/spec/epub-overview-20140228.html
A diff of changes from the previous version is also available.
Copyright © 2010-2013 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, International Digital Publishing Forum (IDPF)
William McCoy, International Digital Publishing Forum (IDPF)
MURATA Makoto, JEPA EPUB Study Group
Daniel Weck, DAISY Consortium
This document is informative
TABLE OF CONTENTS
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
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ISO/IEC 23736-1:2020(E)
2.6. Fixed Layouts
2.7. Rendering and CSS
2.8. Multimedia
2.9. Fonts
2.10. Scripting
2.11. Text-to-speech
2.12. Container
3. Global Language Support
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. Content Descriptions
4.5. Aural Renditions and Media Overlays
4.6. Fallbacks
4.7. 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 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 3.0.1 [Publications301], which defines the semantics and overarching
conformance requirements for each Rendition of an EPUB Publication.
EPUB Content Documents 3.0.1 [ContentDocs301], which defines profiles of XHTML, SVG and
CSS for use in the context of EPUB Publications.
EPUB Open Container Format (OCF) 3.0.1 [OCF301], 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 3.0.1 [MediaOverlays301], which defines a format and a processing
model for synchronization of text and audio.
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ISO/IEC 23736-1:2020(E)
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 entirely of informative overview material, including a
roadmap to the four building-block specification documents that compose EPUB 3.
Another informative 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.
› 1.2 Roadmap
This section provides an overview of the EPUB 3 specifications by explaining in brief the components
of an EPUB 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. Each
EPUB Publication consists of one or renderings of its content, called Renditions.
Some of the resources in the EPUB Container facilitate the discovery and processing of the
Renditions, 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 [ContentDocs301].
An EPUB Publication's resources are typically bundled for distribution as a ZIP-based archive with the
file extension .epub. As conformant ZIP archives, EPUB Publications can be unzipped by many
software programs, simplifying both their production and consumption. The container format is
introduced in Container and defined in [OCF301].
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 files of the available Renditions of the EPUB Publication (their Package Documents).
The Package Document is itself a kind of centralized information source for a given Rendition, storing
metadata about the specific work expressed by the Rendition, providing an exhaustive list of
resources and defining a default reading order. The Package Document is introduced in Package
Document and defined in [Publications301].
The preceding components of an EPUB Publication are not new to EPUB 3, and will be familiar to
anyone who has worked with EPUB 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 for a given Rendition. The Overlay
Document is introduced in Multimedia and defined in [MediaOverlays301].
The following example shows the resources a minimal single-Rendition "Hello World" EPUB
Publication might contain:
mimetype
META-INF/container.xml
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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 EPUB 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
Each Rendition of an EPUB Publication includes a single Package Document, which specifies all the
resources required to render that Rendition. The Package Document also defines a reading order for
linear consumption, and associates metadata and navigation information for the Rendition.
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 [Publications301] 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 also includes a collection [Publications301] element, which allows
grouping of logically-related Publication Resources. This element exists to enable the development of
specialized content identification, processing and rendering features, such as the ability to define
embedded preview content, or assemble an index or dictionary from its constituent XHTML Content
Documents.
The Package Document and other Rendition-specific constructs are specified in [Publications301].
› 2.2 Navigation
› 2.2.1 Reading Order
A key concept of EPUB is that an EPUB Publication consists of multiple resources that can be
completely navigated and consumed by a person or program in some specific order.
Many types of publication 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
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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).
Each Rendition of an EPUB Publication defines at least one such logical ordering of all its top-level
content (the spine [Publications301] ), as well as a declarative table of contents (the EPUB Navigation
Document [ContentDocs301]). EPUB 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
Each Rendition of an 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 [ContentDocs301] while not impacting access
to information for accessible Reading Systems.
The structure and semantics of Navigation Documents are defined in EPUB Navigation Documents
[ContentDocs301] .
› 2.3 Linking
The new EPUB Canonical Fragment Identifier (epubcfi) Specification [EPUBCFI] defines a
standardized method for linking into an EPUB 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 metadata. Each Rendition's Package
Document includes a dedicated metadata section [Publications301] for general information about the
EPUB Publication, allowing titles, authors, identifiers and other information about the EPUB
Publication to be easily accessed. It also provides the means to attach complete bibliographic records
using the link element [Publications301] .
The Package Document also allows a Unique Identifier to be established for the EPUB Publication
using the unique-identifier attribute [Publications301] . The required last-modified date in the Package
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metadata section can be joined with this identifier to define a Release Identifier, which provides a
means of distinguishing different versions of an EPUB Publication (see Publication Identifiers
[Publications301] ). The Package Identifier addresses the issue of how to release an EPUB
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 ( XHTML Semantic Inflection [ContentDocs301] ). Both RDFa and Microdata
attributes can also be used in XHTML Content Documents, enabling content-level metadata
expressions ( XHTML Semantic Enrichment [ContentDocs301] ).
› 2.5 Content Documents
Each Rendition of an EPUB Publication contains one or more EPUB Content Documents, as defined
in [ContentDocs301]. These are XHTML or SVG documents that describe the readable content 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
[ContentDocs301] for element-level metadata and the epub:trigger element [ContentDocs301] for
declaratively associating controls with multimedia elements.
These additions do not affect the ability of an HTML5 User Agent [HTML5] to render XHTML Content
Documents, but EPUB Publications might not render identically in all User Agents depending on their
support.
› 2.6 Fixed Layouts
Although EPUB's history is steeped in enabling reflowable content, not all publications lend
themselves easily to reflowing. Children's books, comics and manga, magazines and many other
content forms require the ability to create page-precise layouts to be represented in a meaningful
fashion.
EPUB 3 includes metadata that allows the creation of fixed-layout XHTML Content Documents
[Publications301] , in addition to existing capabilities for fixed layouts in SVG. This metadata enables
the dimensions of the page [ContentDocs301] to be controlled, creating a canvas on which elements
can be absolutely positioned.
The metadata does not just flag whether content is to be fixed or reflowed, but also allows Authors to
specify the desired orientation of pages [Publications301] , when to create synthetic spreads
[Publications301] , and how to position pages [Publications301] within those spreads, providing a
broad range of control over the presentation of EPUB Publications.
› 2.7 Rendering and CSS
A key concept of EPUB is that content presentation adapts 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.
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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 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 might 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 can 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 can
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 might 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 [ContentDocs301] for more information.
› 2.8 Multimedia
EPUB 3 supports audio and video embedded in XHTML Content Document 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 [Publications301] . For
recommendations on embedding video, refer to Reading System Conformance [Publications301] .)
Another key new multimedia feature in EPUB 3 is the inclusion of Media Overlay Documents
[MediaOverlays301]. When pre-recorded narration is available for a Rendition of an EPUB
Publication, Media Overlays provide the ability to synchronize that audio with the text of a Content
Document (see also Aural Renditions and Media Overlays).
› 2.9 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 EPUB 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.
› 2.10 Scripting
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EPUB strives to treat content declaratively — as data that can be manipulated, not programs to be
executed — but does support scripting as defined in HTML5 and SVG (refer to Scripted Content
Documents [ContentDocs301] for more information).
It is important to note, however, that scripting support is optional for Reading Systems and might be
disabled for security reasons.
Authors need to be aware 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 [ContentDocs301] .
In other words, consider limiting scripting to cases where it is 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.
› 2.11 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 [ContentDocs301] for more information.
Inline SSML Phonemes
The incorporation of SSML phonemes functionality [SSML] directly into a EPUB Content
Document [ContentDocs301] 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 [ContentDocs301] 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 Style Sheets [CSS2.1]) enables Authors to control further
speech synthesis characteristics. Refer to CSS 3.0 Speech [ContentDocs301] for more
information.
› 2.12 Container
An EPUB Publication is transported and interchanged as a single file (a "portable document") that
contains the Package Documents, all Content Documents and all other required resources for
processing the included Renditions. The single-file container format for EPUB is based on the widely
adopted ZIP format, and an XML document that identifies the location of the Package Document for
each Rendition in the ZIP archive is located at a pre-defined location within the archive.
This approach provides a clear contract between any creator of an EPUB Publication and any system
which consumes such EPUB Publications, as well as a reliable representation that is independent of
network transport or file system specifics.
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An EPUB Publication's representation as a container file is specified in [OCF301].
› 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 EPUB Publications. The alternate-script property can be
combined with the xml:lang attribute to include and identify alternate script renderings of language-
specific metadata.
Using this property, a Japanese EPUB 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 [Publications301] 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
[Publications301] ).
› 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
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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 might have installed fonts that omit characters
required for other locales, and Reading Systems might utilize intrinsic fonts or font engines that do not
utilize operating system installed fonts. As a result, the text content of an EPUB 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 characters and glyphs unique to an EPUB Publication
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.
Text-to-speech in the Features section for more information on these capabilities.
Refer to
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 w
...
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