Introduction
Reading this document
The EPUB 3 format offers publishers the ability to create advanced reading experiences by leveraging
the features and functionality of the Open Web Platform. The ability to use HTML, SVG, CSS, and
JavaScript enables not only rich and interactive reading experiences but allows content to be
produced for languages regardless of their default horizontal or vertical writing directions. The
choice of these technologies also allows [=EPUB publications=] to be accessibly produced for a wide
range of user needs.
This overview explores, at a high level, the authoring features of EPUB 3 that provide this
functionality. This document is not intended as a tutorial on creating EPUB publications and does
not delve into the technical details of the format or how reading systems operate. Rather, it is
intended for a more general audience seeking to understand the benefits of creating their digital
publications using the EPUB 3 format without having to read through technical specifications.
Before jumping straight into the features, however, it is helpful to have a basic conceptual
understanding of how an EPUB publication is constructed. An EPUB file is effectively just a ZIP file
with some identifying metadata that allows reading systems to recognize and render the publication
inside (the ZIP file is formally called the [=EPUB container=] in the specifications).
One of the key files in this rendering process is called the [=package document=]. It provides the
metadata about the publication, lists all of the resources, and specifies the order in which to
present the [=EPUB content documents=] (i.e., the XHTML and SVG documents that, when rendered,
provide the content).
The other key file is the [=EPUB navigation document=]. This document includes the table of contents
for the publication as well as any other navigation aids the publisher has defined (e.g., a page
list or list of landmarks).
The following figure visually represents the structure of an EPUB
publication:
Image description
'EPUB (OCF) container' as the outer most component which encapsulates the 'EPUB publication'
containing two documents 'Publication Document' and 'EPUB navigation document' along with a
inner component labelled 'publication resources' which contains multiple 'EPUB content
documents' (XHTML, SVG), and multiple 'Other resources' (CSS, png, mp3, mov, etc.).
The body of this document explores these resources in greater detail with the goal of providing
greater insight into the array of features they enable in EPUB 3 publications.
Reading the specifications
For those interested in going beyond the general introduction presented in this document, the EPUB
3.3 specification [[epub-33]] contains all the essential authoring information.
Prior to EPUB 3.3, reading the EPUB 3 specifications could be challenging. The format was split
across multiple documents, and each document interspersed authoring and [=reading system=]
requirements. With the release of EPUB 3.3, however, there is now only one the document to read.
In addition, accessibility requirements for [=EPUB publications=] are described in the EPUB
Accessibility specification [[epub-a11y-11]]. One of the great advantages of a digital format like
EPUB 3 is that it breaks down the accessibility barriers of static print content. When constructed
with accessibility in mind, publications are usable by users with diverse needs.
While reading the Reading Systems specification [[epub-rs-33]] can help understand how content gets
rendered, the technical details of this process are not necessary for authoring content.
For a list of all the recommendation-track EPUB 3 specifications and non-normative notes, refer
to .
Version numbering
An initial point of confusion for people new to EPUB is the difference between "EPUB 3" the format
and the EPUB 3 specification numbering. The current version of EPUB 3 is defined in the EPUB 3.3
specification [[epub-33]]. The minor number attached to the specification (".3") differentiates one
release of the specification from the next, but each new release does not represent a new
format.
The W3C working group that maintains EPUB 3 strives not to invalidate content produced to earlier
versions of the specification, so an EPUB 3 publication produced to the EPUB 3.0 specification, for
example, should also be valid to the EPUB 3.3 requirements.
That is also why this overview is for "EPUB 3", as it typically does not change significantly from
one specification release to the next.
For a general overview of the changes made in each revision, refer to .
The EPUB container
An [=EPUB publication=] is transported and interchanged as a single file (a "portable document") that
contains the [=package document | package document(s)=], the [=EPUB 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, and an XML document that identifies the location of the package document
for the publication 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.
It also ensures that EPUB publications are creatable and editable using widely available ZIP tools.
Publishers have the flexibility to use either commercially available editing applications or build their
own content transformation pipelines.
For more information about an EPUB publication's representation as a container file, refer to Open Container Format (OFC) [[epub-33]].
The package document
Every [=EPUB publication=] is represented by a [=package document=]. The package document specifies all
the resources required to render that representation of the content. The package document also defines a
reading order for linear consumption, and associates metadata and navigation information.
The package document defines a layer on top of the traditional structuring of a typical web site to
facilitate the authoring of digital publications. 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 [[epub-33]] does (i.e., it provides an external declarative means to explicitly
specify navigation through a collection of documents).
The package document also provides a standard way to represent metadata globally
applicable to the content. Each package document includes a metadata section [[epub-33]] for general information about the EPUB publication, allowing
titles, authors, identifiers, and other information to be easily accessed and presented to users. It
also provides the means to attach complete bibliographic records using the [^link^] element
[[epub-33]].
The package document also allows a [=unique identifier=] to be established for the EPUB publication using
the unique-identifier
attribute [[epub-33]]. This identifier provides a way for publishers to differentiate one
publication from another, and even one edition from another.
For more information about the package document, refer to Package
document [[epub-33]].
Navigation
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 a collection of
photographic images is 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 arranged by recipe type).
Each EPUB publication defines at least one such logical ordering of all its top-level content in the
[=EPUB spine | spine=]. Each also defines a declarative table of contents in the EPUB navigation document [[epub-33]]. 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.
Navigation document
Each [=EPUB publication=] contains a special [=XHTML content document=] called the [=EPUB navigation
document=]. This document uses the [[html]] [^nav^] element to define human- and machine-readable
navigation information. All [=reading systems=] make use of the EPUB navigation document to present
a table of contents to their users.
The EPUB navigation document provides baseline accessibility and navigation support and offers
features to enhance navigation for all users. Prime among these are support for internationalization
(for example, as an XHTML document itself, the EPUB navigation document natively supports ruby annotations) and support for embedded grammars (MathML and SVG can be
included within navigation links).
Although EPUB reading systems are not required to support these advanced text formatting features
when they process the data to create their own table of contents, the dual role of the EPUB
navigation document means the features can still be useful for their intended audience. Publishers
can include the EPUB navigation document in the [=EPUB spine | spine=] [[epub-33]] (i.e., as a page
of content in the publication) so that users have direct access to the full HTML.
Including the EPUB navigation document in the body of an EPUB publication also allows publishers
tailor the display differently for users and reading systems. In highly structured documents, for
example, it might not be desirable to display the complete table of contents to users in the body of
the publication. In this case, the display level can be modified using the hidden attribute [[epub-33]]. This
attribute is ignored by reading systems when they render the table of contents outside the [=EPUB
spine | spine=] (e.g., in their own specialized views), which avoids minimizing the information that
is available and improves the overall accessibility.
Publishers can also supply additional [^nav^] elements if their EPUB publications contain
non-structural points of interest, such as to page breaks, landmarks, figures, tables, etc., to
further enhance access to the content.
For more information about the structure and semantics of EPUB navigation documents, refer to EPUB navigation document [[epub-33]].
Content documents
Each [=EPUB publication=] contains one or more [=EPUB content documents=], as defined in EPUB content documents [[epub-33]]. 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 [[html]].
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 many popular web sites now have layouts with less
flexibility.
[=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 CSS style sheets, allowing publishers to define the desired
rendering properties. EPUB 3 follows support for CSS as defined in the [[csssnapshot]].
EPUB 3 also supports CSS styles that enable both horizontal and vertical layout and both
left-to-right and right-to-left writing.
Fonts
EPUB 3 supports two closely related font formats — OpenType [[opentype]] and WOFF [[woff]]
[[woff2]] — 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. Refer to Font Obfuscation [[epub-33]] for more
information.
Scripting
EPUB strives to treat content declaratively — as data that can be manipulated, not as
programs to be executed — but does support scripting as defined in [[html]] and [[svg]]. This allows
publishers to build and add dynamic content such as quizzes, interactive games, and so forth.
Scripting in an EPUB publication can create security considerations that are different from scripting
within a web browser, however. For example, typical same-origin policies are not applicable to
content that has been downloaded to a user's local system. For these reasons, scripting is best
limited 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.
It is also important to note that EPUB 3 does not require scripting support in reading systems, and
scripting might be disabled for security reasons. Publishers need to consider how their publications
will remain readable by users before scripting content essential to its understanding.
Refer to Scripting [[epub-33]] for more information
on scripting capabilities.
Semantic enrichment
[=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.
Publishers can use both RDFa [[rdfa-in-html]] and Microdata [[microdata]] attributes for this
purpose, and also embed JSON-LD metadata records [[json-ld]] in the [[html]] [^script^] element.
Structural semantics
EPUB 3 also includes the [^/epub:type^] attribute [[epub-33]] for adding additional information to
any element in an [=EPUB content document=] to express its purpose and meaning within the work.
Although these semantics are primarily for use in publishers' internal workflows, they do provide
enhanced [=reading system=] behaviors in some cases (specifically, for pop-up footnotes).
Refer to Expressing structural
semantics [[epub-33]] for more information.
Layouts
Reflowable layouts
At its core, EPUB is designed for dynamic layout: content is typically intended to be formatted on
the fly rather than being typeset in a paginated manner in advance. This core capability is useful
for optimizing rendering onto different-sized device screens or window sizes, and it facilitates and
simplifies content accessibility.
EPUB 3 also offers metadata allowing publishers to specify their preferred rendering of reflowable
content. Supporting reading systems could offer users a scrolled view of a reflowable document, for
example, instead of paginating it.
For more information, refer to Reflowable layouts
[[epub-33]].
Fixed layouts
Although EPUB's history is steeped in enabling reflowable content, not all publications lend
themselves easily to reflowing. Page-precise layouts are required to meaningfully represent
children's books, comics and manga, magazines, and many other content forms.
EPUB 3 includes metadata that allows the creation of fixed-layout XHTML content documents [[epub-33]], in addition to existing capabilities for
fixed layouts in SVG. This metadata enables the control of the page dimensions [[epub-33]], 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
publishers to specify the desired orientation of
pages [[epub-33]], when to create synthetic
spreads [[epub-33]], and how to position
pages [[epub-33]] within those spreads, providing a broad range of control over the presentation
of EPUB publications.
For more information, refer to Fixed
layouts [[epub-33]].
Accessibility
A major goal of EPUB is to facilitate content accessibility, and a variety of features in EPUB 3 support
this requirement. More specifically, the EPUB Accessibility specification [[epub-a11y-11]] provides a
framework for evaluating the accessibility of [=EPUB publications=]. It leverages the extensive work
done to define web content accessibility in the Web Content Accessibility Guidelines [[wcag21]], an
internationally recognized standard for content accessibility.
The specification defines requirements to produce [=EPUB publications=] that can be accessed by a wide
range of users. It is accompanied by a techniques document [[epub-a11y-tech-11]] that outlines best
practices for meeting these requirements.
It is important to note that while accessibility is important in its own right, accessible content is
also more valuable content: an accessible EPUB publication will be adaptable to more devices and be
easier to reuse, in whole or in part, via human and automated workflows.
Some specific content features of EPUB 3 that enable the creation of highly accessible content
include:
- HTML semantic markup
-
Compared to EPUB 2, the version of HTML supported in EPUB 3 incorporates a number of new elements
that make markup more semantically meaningful. These include, for example, the [[html]]
[^section^], [^nav^], and [^aside^] elements. These additions allow content to be better grouped
and defined, both to represent the structure of documents and to facilitate their logical
navigation.
- HTML multimedia elements
-
The [[html]] [^audio^] and [^video^] elements come with built-in controls that
work with assistive technologies. These new elements allow users to control playback, no longer
requiring plugins with poor accessibility support.
- Reflowable layouts
-
The default for EPUB publications is to reflow to fit the available screen space. This allows
users to change the display to their liking (e.g., increasing font size, line spacing, and word
spacing) without causing a publication to become unreadable — the resized content
re-adapts itself to the available space.
- ARIA support
-
EPUB includes support for expressing ARIA roles, states, and properties both in XHTML and SVG
content documents. These attributes allow publishers to improve the accessibility of scripted
controls and components.
New and experimental features
EPUB 3 is continuously evolving with the web to adapt new features and technologies. The specification
references a number of web standards that automatically becomes valid to use as soon as they change.
HTML, for example, is now called a "living standard" because new features are added once there are at
least two implementations. EPUB 3 also references versions of SVG [[svg]], JavaScript [[ecmascript]],
and Unicode [[unicode]] that adapt to new releases, and the CSS reference is tied to the latest snapshot
[[csssnapshot]].
What this means for EPUB 3 is that as new features are added to these standards, they are also
immediately valid to use in [=EPUB publications=]. (That does not mean it is always wise to use such
features right away, as [=reading systems=] often take longer to implement support, and vendors may not
always allow the new features past their validation routines.)
The EPUB Working Group may also introduce new and experimental features from time-to-time outside of the
core specifications. This is typically done because the features do not have sufficient real-world
support in reading systems yet, or because the Working Group is looking for implementation feedback
before trying to standardize the technology.
There are currently two such experimental features: multiple-rendition publications
[[epub-multi-rend-11]] and text-to-speech enhancements [[epub-tts-10]]. Publishers may use these
features in their publications but should do so with the awareness that there may be limited
support.
EPUB revision history
OEB, OCF and EPUB 2: 1999—2010
EPUB has 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 renamed EPUB 2.0 in October 2007 and approved in September 2010. This
revision consisted of a triumvirate of specifications: Open Package Format (OPF), Open Publication
Format (OPF), and OCF. EPUB 2.0.1, which was a maintenance update to the 2.0 specification set,
primarily intended to clarify and correct errata in the specifications. See [[opf-201]] [[ops-201]]
[[ocf-201]].
EPUB 3.0: 2010
Work on a major revision of the EPUB specifications began in 2010, with the goal of aligning EPUB
more closely with HTML, and in the process bringing new, native multimedia features, sophisticated
CSS layout rendering and font embedding, MathML support, scripted interactivity, vertical writing
and other enhanced global language support, and improved accessibility. A new specification for EPUB
Media Overlays was also introduced, allowing for 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.
See [[epubpublications-30]] [[epubcontentdocs-30]] [[ocf-30]] [[epubmediaoverlays-30]]
[[epubchanges-30]].
EPUB 3.0.1: 2014
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 publishers greater control
over presentation when a reflowable EPUB is not suitable for the content.
See [[epubpublications-301]] [[epubcontentdocs-301]] [[ocf-301]] [[epubmediaoverlays-301]]
[[epubchanges-301]].
EPUB 3.1: 2017
EPUB 3.1 was the first minor revision of EPUB 3. The goal of this revision was to better align EPUB 3
with current web standards. References to important standards were made undated, meaning that
whenever they are updated they are legal to use in EPUB 3 content (e.g., the latest version of HTML
is always valid to use; a revision of EPUB is not needed). The use of CSS was also clarified, and
the use of EPUB-specific properties reduced.
Many EPUB-specific features were also removed from the standard, in particular content switching,
triggers, and bindings. This change necessitated a new package document version number.
See [[epub-31]] [[epubpackages-31]] [[epubcontentdocs-31]] [[ocf-31]] [[epubmediaoverlays-31]]
[[epubchanges-31]]
EPUB 3.2: 2018
The work on EPUB 3.2 was undertaken shortly after EPUB 3.1 to restore compatibility of content to
EPUB 3. The change of version number introduced in EPUB 3.1 meant that publishers, vendors and
reading system developers would have to produce, distribute, and consume two versions of EPUB
content, but the costs of this change outweighed the benefits of the new version. EPUB 3.2 instead
keeps all the best parts of EPUB 3.1 but deprecates elements instead of removing them so that a new
version number is not necessary in the package document. See [[epub-32]] [[epubpackages-32]]
[[epubcontentdocs-32]] [[ocf-32]] [[epubmediaoverlays-32]] [[epubchanges-32]]
EPUB 3.3: 2022
The work on EPUB 3.3 [[epub-33]] was undertaken in 2020-21, and is the first version of the EPUB 3
series published as a W3C Recommendation. EPUB 3.3 does not include any significant technical change
to, and is strongly backward compatible with, EPUB 3.2. This means that any valid EPUB 3.2
Publication is also a valid EPUB 3.3 Publication.
The documents themselves have been restructured. The primary motivation for this restructuring, as
well as an extensive editorial revision, was to make the documents more readable. Also, as part of
the thorough testing regime developed by the W3C EPUB 3 Working Group, this
restructuring led to the separation of Recommendations and Working Group Notes (see also the detailed list of documents ). Features specified in the
Recommendations are thoroughly tested, are widely implemented in reading systems, and they can be
considered as interoperable. On the other hand, features specified in Working Group Notes, although
they may have some authoring uptakes, still lack support in reading systems; as a result, these
technologies should not yet be considered stable and interoperable.
The separate section in [[epub-33]] provides a more detailed
overview of the changes.