3 Discovery Metadata Techniques
META-002: Identify sufficient access modes
The access modes sufficient to consume an EPUB Publication express a broader
picture of the potential usability than do the basic access modes. Where
the basic access modes identify the default nature of the media used in the publication, sufficient access
modes identify which modes can be used to read the entire publication. Sufficient access modes also take into
account the adaptations that have been provided, allowing a User to determine whether the content will be
usable regardless of its default nature.
Sufficient access modes are identified in the [schema.org] accessModeSufficient property. Repeat the property for each set of sufficient access
modes.
For example, consider an EPUB Publication that contains graphics and charts, as well as descriptions for
all these images. The publication has both textual and visual access modes, so the Author will include the following metadata entries to indicate this:
This metadata does not make clear that a textual access mode is sufficient to read the entire publication,
or that a visual one is, only that two modes are required by default. This discrepancy is why sufficiency is
also important to know.
The first set of sufficiency metadata the Author inputs will establish the textual and visual
requirement:
Since the Author has also included descriptions for all the images, she can also indicate that a purely
textual access mode is sufficient to read the content:
Users now have a more complete idea of the readability of the content.
The full set of entries in this example is as follows:
Note that sufficiency of access is often a subjective determination of the Author based on their
understanding of what information is essential to comprehending the text. Some information loss occurs by not
being able to view a video, for example, but the Author might regard the visual or auditory losses as
inconsequential if a transcript provides all the necessary information to understand the concepts being
conveyed.
Refer to the accessModeSufficient proposal for more information about this property and its values.
4 WCAG Techniques
4.1 General Guidance
Techniques for meeting the requirements of the Web Content Accessibility Guidelines (WCAG) 2.0 [WCAG 2.0] are defined in Techniques for WCAG 2.0 [WCAG 2.0 Techs]. This
document does not repeat these techniques.
In general, the differences between the application of WCAG techniques to Web pages and their application
to EPUB Content Documents is minimal, but the following sections
outline some key differences.
One point to note is that the WCAG techniques cover a greater range of technologies and content types than
are typically found in an EPUB Publication, so many are not applicable.
The following sets of techniques are the most broadly applicable to EPUB Content Documents:
Other techniques will apply depending on the technologies used (e.g., a [SWF] video in
EPUB 2) or any alternative formats embedded in the EPUB Publication (e.g., a PDF form).
Helpful Resources
Authors not familiar with [WCAG 2.0 Techs] may find
the number of techniques daunting, as they are intended to provide broad coverage of possible
solutions.
Assistance applying these techniques to EPUB Content Documents
is available from the following sources:
4.2 Content Access
ACCESS-001: Ensure linear reading order of the publication
[WCAG 2.0] Success Criterion
1.3.2 specifies that each Web page have a meaningful order, but the content of an EPUB Publication typically spans multiple documents. It is consequently essential
not only that each EPUB Content Document has a meaningful order, but that the order is
meaningful from document to document.
Authors need to ensure that all EPUB Content Documents are included
in the spine [Packages 3] and put in sequence so that the reading order is preserved.
Authors also need to ensure that they identify whether items in the spine contain primary or
supplementary information using the linear
attribute [Packages 3] so that the Reading System can optimally present
such content.
ACCESS-002: Provide multiple ways to access the content
[WCAG 2.0] Success Criterion
2.4.5 requires there be more than one way to locate a Web page within a set of Web pages. By
default, EPUB Publications meet this WCAG requirement so
long as Authors follow the EPUB requirements to include all EPUB Content
Documents in the spine and ensure access to all
non-linear documents [Packages 3].
The reason an EPUB Publication passes by meeting these requirements has to do with differences in how a
User interacts with the set of documents in an EPUB Publication. In
particular, although an EPUB Publication typically consists of many Content Documents, Reading Systems automatically
provide the ability for the User to move seamlessly from one document to the next, so long as they are
listed in the spine [Packages 3]. To the User, an EPUB Publication is a single
document they have complete access to, not a set of disconnected pages that they need links to move
through.
The required table of contents provides a second method to access the major headings of the
publication. The User can jump to any heading and continue to navigate from there, regardless of how the
publication is chunked.
Following these two requirements therefore satisfies the need for multiple ways to access the content.
Reading Systems also typically provide search capabilities, something the Author cannot provide, so Users
also have a third option available in most cases.
Although Authors only need to follow EPUB requirements to meet this criteria, they are still encouraged
to provide additional methods to improve access beyond the minimum. Some suggestions include:
-
adding at least one link to every Content Document in the spine to the table of contents, when
feasible;
-
adding an index to locate major topics;
-
adding additional navigation aids to the EPUB Navigation Document (e.g., lists of
figures and tables).
1 Note About the Table of Contents
A common question about the EPUB table of contents is what completeness it needs to have with
respect to the headings of the publication. Although the obvious answer seems like it should be a
simple aggregation of all headings for all sections, practically there are a number of usability
challenges to this approach.
Factors such as device screen sizes can make the table of contents for publications with a deep
hierarchy of headings unreadable, so Authors trim headings below
a certain depth to improve the quality. Reading Systems
also do not always provide structured access to the headings in the table of contents, or shortcuts
to move easily through the headings, leaving Users that have them
read aloud navigating the links one at a time.
Although it is expected that Reading Systems will improve access to the table of contents as
accessibility support for EPUB evolves — making complete tables of contents usable by everyone —
there are legitimate usability reasons why they are not provided now.
When Authors choose not to provide links to all the headings, however, they should optimize the
linking they do provide for the best overall reading experience. Some considerations on how to
achieve this include:
-
ensuring that there is at least one link to every EPUB Content Document —
allowing the User to reach each document simplifies navigation to the minor headings within
them;
-
only omitting minor headings from the table of contents — although a subjective decision,
there is often a level of diminishing value for navigation (e.g., fourth level and lower
headings often only delimit short subsections on a topic).
4.3 Semantics
SEM-001: Include ARIA and EPUB semantics
The [WAI-ARIA 1.1] role
attribute is used to provide
additional semantic information about the host markup to Assistive
Technologies — Authors can attach it to any element. The
use of roles allows Assistive Technologies to automatically scan the markup and compile a list of landmarks for Users, enabling quick
access to key features of the content.
The role attribute is similar in nature to the type attribute [Content Docs], which provides
additional semantic information to enable EPUB Reading System behaviors.
The key difference between these attributes is that the role attribute bridges
accessibility while the type attribute provides hooks to enable Reading System
behaviors. Omitting roles lessens the accessibility for Users of Assistive Technologies, in other words,
while omitting types diminishes certain functionality in Reading
Systems (e.g., pop-up footnotes or special presentations of the content).
Since each attribute offers different advantages, it is recommended that both be used whenever
applicable to provide the best reading experience for all readers.
It is often the case that the attributes will have have corresponding semantics. In these situations,
it is recommended that both attributes be attached to an element.
Helpful Resources
The following documents list the semantics that are available for use with each attribute:
-
The Digital Publishing WAI-ARIA Module [DPUB Vocab] provides a set of
core publishing roles, but any roles from [WAI-ARIA 1.1] can be used in the
role attribute.
-
The EPUB Structural Semantics Vocabulary [Structure Vocab] provides
the default values that can be used with the type attribute, but the
attribute is extensible to allow semantics from other vocabularies.
The following documents explain the application of ARIA roles for Authors already familiar with the
use of the EPUB 3 type attribute for semantic inflection:
SEM-002: Do not repeat semantics across chunked content
Although EPUB Publications appear as single contiguous
documents to Users when read, they are typically composed of many
individual EPUB Content Documents. This practice keeps the
amount of markup that has to be rendered small to reduce the load time when the User cannot interact with
the documents. It is rare, at least for books, that an EPUB Publication contains only one Content
Document with all the content in it.
When content is chunked in this way, it often requires the Author to make decisions about how best to
restructure the information. A part, for example, cannot contain all the chapters that belong to it. The
Author instead has to flatten the structure and separate the part heading from each chapter.
Although visually these restructuring decisions can be hidden from readers, they impact the
functionality of Assistive Technologies. In the case of [WAI-ARIA 1.1] roles, the result is that only the subset present in the currently-loaded EPUB
Content Document are exposed to Users. An Assistive Technology cannot
provide a list of landmarks for the whole publication, as it cannot see outside the current
document.
To counteract this destructuring effect, Authors sometimes think to
re-add or re-identify structures in the belief that having this information in every document will be
helpful to Users (e.g., adding an extra [HTML] section element around
a chapter to indicate it belongs to a part, or putting the part semantic on the
body tag). All this practice does, however, is add repetition that is not only
disruptive when reading, but can actually make the structure of the publication harder to follow. Authors
are therefore advised not to attempt to rebuild structures in these ways.
For example, consider a book that has five parts and each part contains five chapters. Structurally,
each chapter belongs to its part (i.e., is grouped with it), as in the following markup:
Since this would lead to a large content file, the part heading is typically split out into its own
Content Document so that it will appear on its own page:
Each chapter is then separated into a separate Content Document:
It is not necessary to add a part wrapper to each chapter, as in the following example:
Doing so introduces a new part landmark into each document, which will cause an
Assistive Technology to inform the User that the landmark is available to navigate to.
SEM-003: Include EPUB landmarks
[WAI-ARIA 1.1] landmarks are similar in nature to EPUB landmarks [Packages 3]: both are designed to provide Users
with quick access to the major structures of a document, such as chapters, glossaries, indexes, etc. ARIA
landmarks are compiled automatically by Assistive Technologies from the
roles that have been applied to the markup, so Authors only need to follow the requirement to include roles for the
landmarks to be made available to Users.
Although automatic generation of ARIA landmarks simplifies authoring, it also means that ARIA landmarks
are limited to how the EPUB Publication has been chunked up into Content Documents. An Assistive Technology can only present the
landmarks available in the currently-loaded document; it cannot provide a complete picture of all the
landmarks in a multi-document publication (see the previous section
for more discussion about content chunking).
EPUB landmarks, on the other hand, are statically compiled by the Author prior to distribution, and are
not directly linked to the use of the type attribute [Content Docs] in the content. They are designed to
simplify linking to major sections of the publication in a machine-readable way, as Reading Systems do not scan the entire publication for
landmarks, either. EPUB landmarks are typically not as numerous as ARIA landmarks, as Reading Systems
only expose so many of these navigation aids.
Given these differences in application, however, it is important to include EPUB landmarks and not rely
only on the presence of ARIA roles to facilitate navigation, and vice versa. Each aids navigation in its
own way.
The EPUB specification does not require that Authors include a specific set of landmarks, but it is
recommended to include a link to the start of the body matter as well as to any major reference sections
(e.g., table of contents, endnotes, bibliography, glossary, index).
Helpful Resources
The following resources explain EPUB and ARIA landmarks in more detail.
4.4 Titles and Headings
TITLES-001: Include publication and document titles
[WCAG 2.0] Success Criterion
2.4.2 requires that each Web page include a title. EPUB has a similar requirement for EPUB Publications: publications require a [DCMES] title element in the Package Document
metadata. The [WCAG 2.0] requirement is not satisfied by the EPUB requirement,
however.
When authoring an EPUB Publication each EPUB Content Document also requires a descriptive
title that describes its content. If not provided, Assistive Technologies
often will announce the name of the file to Users.
If the title includes structural context (e.g., the part heading a chapter belongs to, or the name of
the publication), order the title such that the most precise description of the current document comes
first.
For more information about titles, see Technique H25 [WCAG 2.0 Techs].
TITLES-002: Ensure numbered headings reflect publication hierarchy
To a User, an EPUB Publication appears as a single
document that they read from beginning to end, even though the content is often split across numerous
EPUB Content Documents. As a result, their natural
expectation is that the headings reflect their position in the overall hierarchy of the publication,
despite the publication not actually being a single document (e.g., if a part heading is expressed in an
[HTML] h1 element, each chapter that belongs to the part will
have an h2 heading).
[WCAG 2.0 Techs] G141:
Organizing a page using headings instructs Authors on
correctly using numbered headings within a document, but with EPUB Publications the numbered headings
also need to remain consistent across documents. Practically, this means that each EPUB Content Document
does not have to begin with an h1 heading unless the first heading is a top-level
heading — the first heading needs to have a numbered heading element relative to its actual
position.
Authors also to need chunk their content so that the first heading in a document always has the highest
number. For example, if a document starts with an h3 heading, there shouldn't be
an h2 heading later in the document (e.g., do not include the start of a new section
with the trailing subsections of the previous). It is acceptable for their to be subsequent headings at
the same level as the first (e.g., multiple subsections in one document could all have
h3 headings).
4.5 Descriptions
DESC-001: Include alternative text descriptions
[WCAG 2.0] Success
Criterion 1.1.1 requires a text alternative for all non-text content. In the case of image
content, the provision of alternative text (e.g., in an [HTML] alt attribute) is sufficient for meeting the requirements of the [EPUB Accessibility] specification.
Authors are strongly encouraged to include extended descriptions,
but the utility of extended descriptions often depends on the needs of the User.
Until the publishing ecosystem matures, making it easier for Authors to include extended descriptions and
write them to optimally meet the needs of the greatest number of Users, they are not required.
If extended descriptions are not included, note the omission in the accessibility summary.
note
The requirement for a text alternative does not apply to decorative images that contribute no
information to the text. These are identified by an empty alt attribute.
caution
This guidance applies to the evaluation of EPUB
Publications strictly for achieving conformance to the [EPUB Accessibility] specification. If an EPUB Publication is distributed in an environment where extended descriptions
are required, this guidance for conformance will not be sufficient.
note
The use of alternative text as a sufficient description for all images may be removed in a future
version of these techniques.
Helpful Resources
The following documents provide guidance on including extended descriptions:
DESC-002: Include descriptions for MathML content
Reading Systems typically do not provide self-voicing of [MATHML] markup, even when
the equations can be rendered visually. Although polyfill scripts such as MathJax provide the ability to visually render MathML
equations when scripting is supported, they do not provide an accessible alternative for non-visual
readers. As a result, the inclusion of MathML markup alone is not sufficient to make equations accessible
to non-visual readers.
EPUB 3 recommends the use of the altimg
attribute to include a fallback image [Content Docs] for Reading Systems that cannot render the equations, but a visual
fallback alone is not sufficient for all readers.
Due to these rendering limitations, inclusion of a description is recommend to complement the markup
and also provide a text-accessible alternative.
A short description can be provided using the [WAI-ARIA 1.1] aria-label attribute on an [HTML] div or span element surrounding the MathML
markup. For lengthier descriptions, use the aria-describedby
attribute attribute.
note
Although the [MATHML] alttext attribute provides similar
descriptive capabilities, it is not as well supported by Assistive
Technologies as the ARIA attributes. If support increases in the future, this technique
will be updated to also cite it as an option. Authors are not
discouraged from also including the attribute, or from using it for other purposes (e.g., storing the
description with its equation in content management systems).
4.6 Accessible Alternatives
As EPUB Publications can be composed of more than one Rendition, it is possible that different versions of the content will have different
levels of accessibility. For example, an image-based version of the content that lacks alternative text or
descriptions could be bundled with a WCAG-compliant text-based serialization. This type of accessible
bundling is acceptable, as [WCAG 2.0] allows non-conforming content provided a conforming alternate
version is available.
The [Multiple Renditions] specification defines a set of features for creating these types of
EPUB Publications. In particular, it specifies a set of attributes that allow a Reading System to automatically select a preferred Rendition for the User, or to provide the User the option to manually select between the available
options. This functionality technically meets the requirements of [WCAG 2.0] in terms of
ensuring the User can access the accessible version.
In practice, however, the [Multiple Renditions] specification is not broadly supported in
Reading Systems at the time of publication. As a result, a User who obtains an EPUB Publication that contains
more than one Rendition will only have access to the Default Rendition. Unless this
Rendition is the accessible one, the EPUB Publication might not be readable by them.
Authors therefore need to use their best discretion when implementing
this functionality to meet accessibility requirements. EPUB Publications that contain multiple renditions are
conformant to the [EPUB Accessibility] specification if at least one Rendition meets all the content
requirements, but Authors at a minimum need to note that a Reading System that supports multiple renditions
is required in their accessibility summary. Any other methods the Author
can use to make this dependence known is advisable (e.g., in the distribution
metadata).
This section will be updated with techniques for using multiple renditions when there is enough support in
Reading Systems to broadly recommend their use.
Acknowledgements and Contributors
This section is informative
EPUB has been developed by the International Digital Publishing Forum in a cooperative effort, bringing together
publishers, vendors, software developers, and experts in the relevant standards.
The EPUB Accessibility specification and techniques were prepared by the International Digital Publishing Forum’s
EPUB Maintenance Working Group, operating under a charter approved by the membership in July 2015, under the
leadership of:
- Gylling, Markus (International Digital Publishing Forum (IDPF)) Chair
- Siegman, Tzviya (John Wiley & Sons) Chair
- Conboy, Garth (Google Inc.) Vice-chair
- Duga, Brady (Google Inc.) Vice-chair
Work on these documents was undertaken by an accessibility subgroup led by:
- LaPierre, Charles (
Benetech
)
- Singh, Avneesh (
DAISY Consortium
)
Active members of the working group included:
IDPF Members
- Albert, Paolo (PubCoder)
- Baker, Mike (Houghton Mifflin Harcourt)
- Balakrishnan, Ramu (MPS Limited)
- Cho, Yong-Sang (KERIS (Korea Education and Research Information Service))
- Comerford, Rachel (Macmillan)
- Cramer, Dave (Hachette Book Group)
- Creasy, Keith (American Printing House for the Blind, Inc.)
- Deltour, Romain (DAISY Consortium)
- Geffroy, Jean-Marie (Mantano)
- Gusso, Lina (Pearson)
- Gylling, Markus (International Digital Publishing Forum (IDPF))
- Hanson, Lauren (Kaplan Publishing)
- Harris, Eric (VitalSource/Ingram Content Group)
- Heinser, Bernhard (Access for All)
- Johnson, Rick (VitalSource/Ingram Content Group)
- Kasdorf, Bill (Apex CoVantage)
- Keating, Patrick (Bluefire Productions)
- Kennedy, Naomi (Penguin Random House)
- Kerscher, George (DAISY Consortium)
- LaPierre, Charles (Benetech)
- Leo, Arthic (Amnet Systems)
- Mietkiewicz, Elie (Gutenberg Technology)
- Murata, Makoto (JEPA EPUB Study Group)
- Mussinelli, Christina (Associazione Italiana Editori)
- Prabhu, John (HOV Services)
- Siegman, Tzviya (John Wiley & Sons)
- Singh, Avneesh (DAISY Consortium)
- Soiffer, Neil (Design Science)
- Thurston, Jonathan (Pearson)
- Webster, Amaya (Benetech)
- Weck, Daniel (DAISY Consortium)
Invited Experts/Observers
- Andrews, David
- Bonelli, Giuseppe
- Garrish, Matt
- Kudou, Tomoyuki
- Leva, Paolo
- Marquès Solé, Daniel
- McGlone, Jonathan
- Morris, Julie
- Munchenburger, Manfred
- Rasmussen, Lloyd
- Reddy, Ramesh
- Rothberg, Madeleine
- Sheehan, John
- White, Jason
For more detailed acknowledgements and information about contributors to each version of EPUB, refer to Acknowledgements and Contributors [EPUB3 Overview].
