EPUB Accessibility Techniques defines content accessibility requirements for EPUB® Publications.
Status of This Document
This section describes the status of this
document at the time of its publication. A list of current W3C
publications and the latest revision of this technical report can be found
in the
W3C standards and drafts index.
This document, EPUB Accessibility Techniques, provides informative guidance on how to apply the
accessibility requirements defined in the EPUB Accessibility 1.2 specification [epub-a11y-12] that
are unique to EPUB publications.
This document does not cover general web accessibility techniques already addressed in [wcag2] and
[wai-aria], for example, for which no substantive differences in application exist. Following
those techniques, as applicable, is also essential to meeting the accessibility requirements of the
EPUB Accessibility 1.2 specification.
This document is not intended to be read in isolation, in other words, as it does not define
conformance requirements for making accessibility claims or cover every method for producing
accessible content. An EPUB publication has to meet all the requirements of EPUB Accessibility 1.2
to make a claim of accessibility; verifying only the techniques in this document is not
sufficient.
Only the first instance of a term in a section links to its definition.
2. About the techniques
The accessibility techniques described in this document are advisory in nature. They are intended to help
create EPUB publications that conform to the requirements
in [epub-a11y-12], but they are not all applicable in all situations and there can be other ways to
meet the requirements of that specification. As a result, this document is not intended to be read as
providing prescriptive requirements.
These techniques also do not address issues in digital publishing for which no universally accessible
solutions exist. The W3C's Digital Publishing Interest Group has published a note that outlines many of
these issues [dpub-accessibility]. As solutions become available, they will be incorporated into the
appropriate document, whether this one or one it refers to.
When encountering issues that are not covered in these or related techniques, please report an issue to
the appropriate community for guidance on how to meet accessibility standards. The W3C Web Accessibility Interest Group has a public mailing list
where issues meeting [wcag2] and [wai-aria] requirements can be raised. The W3C Publishing Community Group issue tracker can
be used to ask for support implementing EPUB-specific requirements and the EPUB 3 Working Group's issue tracker to report
issues with this document.
Techniques for meeting the requirements of the [wcag2] are defined in Techniques for WCAG. This document does not
repeat those 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 applicable to EPUB content documents:
3.2.1 Ensure the order of table of contents entries matches linear order
The table of contents provides users more than just links into the content. It is also a means to
understand the structure and ordering of an EPUB
publication. Consequently, users might have difficulty locating where they are in a
publication, where they want to go, and also how to return to previous locations when the order
of entries in the table of contents does not match the linear reading order.
To ensure that the table of contents matches the linear order of the content, the order of its
entries needs to reflect both:
the order of each referenced section within its respective EPUB content document.
Only if there is a logical case for an alternative arrangement of entries is it advised that the
ordering differ. Such scenarios typically only occur when the content does not have to be read
linearly or when additional information is included at the end of a table of contents. For
example, the table of contents for a magazine might be ordered to list all the major articles
first, followed by features, etc.
When the ordering of the table of contents does not match the content, the accessibility summary needs to explain why.
Avoid linking directly to supplementary content from of the table of contents. Instead, define
navigation lists to group together related links, such as a table of figures or a table of
illustrations. These types of lists can be created using custom nav elements [epub-3] in
the EPUB navigation document or using
navigation lists in XHTML content documents. When these navigation lists are included in the
spine, the table of contents can link users to them.
Note
Reading systems do not always provide users an interface to access to custom nav
elements in the EPUB navigation document. Consequently, when using the navigation document
to add supplementary content lists, it is best to ensure the document is also included in
the spine.
3.2.2 Include EPUB landmarks
[wai-aria] landmarks are similar in nature to EPUB landmarks [epub-3]: both are designed to
provide users with quick access to the major structures of a document, such as chapters,
glossaries and indexes.
ARIA landmarks are compiled automatically by assistive technologies from the roles [wai-aria] that have been applied to in an EPUB content document, but
this also means that ARIA landmarks are limited to how the EPUB publication has been chunked up.
An assistive technology can only present the landmarks found in the currently-loaded document;
it cannot provide a complete picture of all the landmarks in a multi-document publication.
EPUB landmarks, on the other hand, are compiled prior to distribution, and are not directly
linked to the use of the type attribute [epub-3] 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. Providing EPUB landmarks further fulfills the requirement for multiple
ways to access the content.
The EPUB specification does not require that EPUB publications include a specific set of
landmarks; it only recommends 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).
3.2.2.1 Helpful resources
The following resources explain EPUB and ARIA landmarks in more detail.
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.
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).
Technique G141: Organizing a page
using headings provides guidance 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 that reflects its actual position in the
publication.
It is advised to chunk EPUB publications so that the first heading in a document always has the
highest number. For example, if a document starts with an h3 heading,
an h1 or h2 heading does not appear 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 there to be subsequent headings at the same level as the first (e.g., multiple
subsections in one document could all have h3 headings).
3.3.3 Heading topic or purpose
Success Criterion 2.4.6 [wcag2] currently states that
all headings have to describe their topic or purpose. The implication of this wording is that
all chapters in a novel, for example, have a topic or purpose and that the topic or purpose is
always clearly reflected by the title of the chapter. Not only is this not always the case, but
this success criterion also complicates the use of chapter numbers as headings since these do
not establish a topic.
By this interpretation, the headings in publications always pass provided they are unique.
It is expected that the wording of the success criterion will be updated to better reflect the
uniqueness requirement, likely in the future WCAG 3 due the complexities of changing the wording
for WCAG 2.
3.4 Descriptions
3.4.1 Include alternative text descriptions
The first version of these techniques only required alternative text for images regardless of
their complexity. This exception is no longer valid.
Image-based content in EPUB publications now has to
meet [wcag2] requirements for alternative text and extended descriptions to conform with
[epub-a11y-12].
3.4.1.1 Helpful resources
The following documents provide guidance on including extended descriptions:
Success Criterions 3.1.1 and 3.1.2 [wcag2] deal with the language of a page and
changes of language with in, respectively.
For EPUB publications, the package document is also an important source of
metadata information about the publication. For example, reading systems expose details of the
publications to users in their bookshelves using this information.
Consequently, it is necessary to provide the language of all text content in the package document
to conform with these WCAG success criteria. The easiest way to meet this requirement is to add
an xml:lang attribute on the root package element [epub-3].
If individual metadata fields within the package document are expressed in a different language,
it is similarly required that the language change be identified by an xml:lang
attribute on the element for the field.
Providing this information enables reading systems to correctly render the text content in the
proper language for users.
Note
The languages specified in the package document have no effect on individual EPUB
content documents (i.e., the language of each document has to be specified using the language
expression mechanisms it provides).
Although it is not strictly required to set this information to meet Success Criterion 3.1.1 [wcag2], as it is
non-normative, it is best practice to always set this field with the proper language
information. (Note that EPUB3 requires the language always be specified, so omitting will fail
validation requirements.)
Although reading systems do not use this
language information to render the text content of the EPUB publication, they do use it to
optimize the reading experience for users (e.g., to preload text-to-speech engines so users do
not have a delay when synthesizing the text).
Note
The languages specified in the package document have no effect on individual EPUB content documents (i.e., the language
of each document has to be specified using the language expression mechanisms it provides).
3.6 Text
3.6.1 Use Unicode for text content
Success Criterion 1.1.1 [wcag2] requires that text
equivalents be provided for all non-text content to meet Level A.
In some regions (e.g., Asia), it is not uncommon to find images of individual text characters,
despite the availability of Unicode character equivalents. This practice occurs for various
reasons, such as ease of translation of older documents and for compatibility across reading systems. The use of images in most
instances leads to the text not being accessible to non-visual users, however.
When individual characters are replaced by images, there is invariably a negative effect on
text-to-speech playback, even when alternative text is provided (e.g., if single characters
within a word are replaced by images, the word will not be read as a single unit of text). It is
also problematic for visual users, as the images often scale poorly and the characters cannot be
changed to different font families to meet user preferences.
The use of Unicode characters for all text content avoids this problem, allowing content to
successfully meet the minimum requirement for Level A.
For compliance with Level AA, please also refer to Success Criterion 1.4.5 which further restricts the use
of images of text to only a set of essential cases.
3.7 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 [wcag2] allows non-conforming content provided a conforming alternate version is
available.
The [epub-multi-rend-11] specification defines a set of features for creating these types of EPUB
publications. 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 [wcag2] in terms of ensuring the
user can access the accessible version.
In practice, however, the [epub-multi-rend-11] 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. Unless this rendition is the
accessible one, the EPUB publication might not be readable by them.
Using multiple renditions to meet accessibility requirements is only advised when there is no other
alternative. EPUB publications that contain multiple renditions are conformant to the
[epub-a11y-12] specification if at least one rendition meets all the content requirements, but
such publications, at a minimum, need to note that a reading system that supports multiple
renditions is required in their accessibility summary. The use of methods outside the EPUB publication that can make this
dependence known are also 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.
4. EPUB Techniques
4.1 Page navigation
4.1.1 Provide page break markers
Both the EPUB Structural Semantics Vocabulary [epub-ssv] and Digital Publishing WAI-ARIA 1.0
Module [dpub-aria-1.1] include a semantic for static page breaks: pagebreak and doc-pagebreak, respectively.
For accessibility purposes, it is most important to specify the role attribute value
as this is what assistive technologies
will process. The epub:type attribute is not recognized by these devices, but it
could be used by epub reading systems to
enhance the user experience. For compatibility with both types of devices, it is advised to
apply both attributes, although only the role attribute is strictly required for
accessibility conformance.
A title or aria-label attribute is required on the element, as it
provides the value that is announced to the user.
The page number can be inserted as text content within the element, but assistive technologies
might not provide a way to disable announcing of the numbers. As a result, users might hear the
numbers announced wherever they occur, which could cause confusion (e.g., the number might seem
to be part of the sentence it occurs within).
EPUB 2 does not include markup to identify static page break marks in the content. Page break
destinations can be included to enable hyperlinking, but the page
list is the only way a user can jump to the locations.
Do not use the [html] a element to identify page break
locations in EPUB 3 publications. Although this element was previously defined as the anchor for
a hyperlink destination, its purpose has been changed in [html] for use solely as a link.
Do not use the deprecated value printPageNumbers. Page break markers do not
always correspond with a print source.
4.1.2 Identify pages in audio playback
Readers rarely stop reading to review each new page number, so when page numbers are read aloud
in the audio playback of a publication it is not only distracting, but can be confusing, as well
(e.g., the number could be read out in the middle of a sentence).
To mitigate this potential annoyance to readers, it is advised to identify page announcements in
media overlay documents when they are
included. Identification allows a reading
system to provide a playback experience where the numbers are automatically skipped.
To identify page numbers in media overlay documents, attach an epub:type attribute
with the value "pagebreak" [epub-ssv]
to each par element [epub-3] that
identifies a page number.
Note that as EPUB 2 does not provide text and audio synchronization, this technique does not
apply.
4.1.3 Provide a page list
A page list — a list of hyperlinks to the static page break locations — is the most effective way
for users to find static page locations. Without a page list, the user would have to navigate
each page marker in the text, provided they are available and the reading system provides such
functionality.
When a page list is included, reading systems
can provide users direct access to the list or use it to provide automatic page jump
functionality.
Users typically want to know the source of the page break markers included in an EPUB publication when they are derived from a
static media. Considerations like which printing, by which publisher or imprint, and whether the
pagination comes from the hard or soft cover edition will affect decisions about its usefulness
(e.g., does it exactly match the pagination of a print book used in a classroom).
These techniques previously advised using the source-of property to identify the dc:source element
containing the source of the pagination [epub-3]. This method came with a number of
limitations, however. It was not future proof, for example, as it relied on the EPUB 3 refines attribute [epub-3] and it
could not handle pagination without a source due to the reliance on a dc:source
element.
The pageBreakSource property
[epub-3] replaces source-of in newer version of EPUB 3. It is strongly
advised that anyone using the source-of property switch to using the
pageBreakSource property. Although the source-of method will
remain valid for backwards compatibility purposes, its use is no longer advised.
If an ISBN is not available, include as much information as possible about the source publication
(e.g., the publisher, date, edition, and binding).
If the page break markers are unique to the EPUB publication, use the value none
with the pageBreakSource property.
4.2 Synchronized text-audio playback
4.2.1 Ensuring complete text coverage
Ensuring the complete text of an EPUB publication
is synchronized with audio is key to allowing users who require full synchronized playback, or
even audio-only playback, have access to the same information as users who do not require
synchronized playback.
The primary consideration for this objective is what constitutes the text content of an EPUB
publication. The minimal candidates for synchronization with audio are all the elements with
visible text content.
In HTML, the class of elements called palpable content [html] can typically be synchronized
with audio or have their own built-in audio. Embedded text in SVG documents, on the other hand,
is found in the text
element [svg] and its descendants.
The media overlays text element is used
to reference these elements, either to play back the pre-recorded audio in a sibling audio element [epub-3] or to
initiate playback of an audio or video element if the audio element is missing
(e.g., for embedded audio and video in the document).
Note
Do not synchronize media overlays to hidden text content. Synchronizing audio with invisible
text will be confusing for sighted readers following the playback.
Text content in a collapsed element, like the details
element [html], is not considered hidden content.
In addition to synchronizing the visible text, synchronized text-audio playback also has to
address text alternatives for image-based content. Images can have alternative text and
descriptions that are not visible to all users. As synchronization is also meant to aid users
who cannot see the images, including these text alternatives and descriptions in the playback is
essential to providing the user all the information in the EPUB publication.
Text alternatives and descriptions in HTML can be represented in the alt attribute [html] and linked by ARIA
attributes (e.g., aria-describedby and
aria-details [wai-aria]).
Descriptions for image elements in SVG are typically represented in a desc element but
ARIA attributes can also be used.
4.2.2 Specifying the reading order
The default reading order typically represents the order in which reading systems render content to users during
synchronized text-audio playback. For EPUB
publications, this is a combination of the sequence of EPUB content documents in the spine and the
order of elements within each EPUB content document.
If there are cases where the logical reading order (how a reader would naturally read the
content) diverges from the default reading order, order the playback sequence of seq and par elements in a media overlays document [epub-3] to match the
logical order.
Use caution when making alterations, however, as other accessibility issues can arise when the
logical order does not match the default order. For example, the content might not be accessible
to users of assistive technologies when
the order in the markup does not match how the assistive technology reads the content. In these
cases, using playback to create a logical order can make the EPUB publication fail WCAG
conformance requirements.
One case where the logical order can diverge from the reading order and remain accessible is in
tables, as assistive technologies typically allow users to choose whether to read by row or by
column.
4.2.3 Identifying skippable structures
Some content elements are not critical to read when following the primary narrative of a work,
and that would interrupt a user's concentration if they had to stop and listen to. Footnotes and
endnotes are examples of such content, as users might only want to come back and read this
content after finishing the EPUB publication. The
announcement of page break numbers can be similarly annoying to readers.
EPUB 3's media overlays feature [epub-3] does not
allow reading systems to determine if playback
sequences are skippable unless additional semantics are added to the markup using the epub:type attribute [epub-3] on
seq and par elements [epub-3]. These
semantics are what allow reading systems to provide users the option to skip their playback
sequences.
It is strongly advised to identify the following structures for skippability:
Endnotes — use the endnotes and
endnote semantics [epub-ssv-11]
for groups of notes and individual endnotes, respectively.
Footnotes — use the footnotes
and footnote semantics [epub-ssv-11]
for groups of footnotes and individual footnotes, respectively.
The identification of other structures is encouraged but is not necessary to meet this
requirement.
4.2.4 Identifying escapable structures
Some content elements are containers for expressing complex information. A table, for example,
has data arranged in rows and cells. Lists similarly can contain many items. While users might
be interested in some of the information in these structures, they also often want to escape
from them to keep reading, not have to listen to the entire content before being able to move
on. These are called escapable elements, because the user needs to be able to escape from them
whenever they choose to simplify the reading experience.
The identification of other structures is encouraged but is not necessary to meet this
requirement.
Note
Identifying nested escapable structures is not advised at this time. Refer to Escapability [epub-3] for more information.
4.2.5 Synchronizing the navigation document
A media overlay document can be provided for
the EPUB navigation document even when the
navigation document is not included in the spine. Doing
so allow reading systems to announce the link
labels regardless of how they present the navigation elements to users (e.g., many reading
systems applications create custom table of contents panels by extracting the data from the EPUB
navigation document).
The process for adding a media overlay document is no different than one for any other
document.
5. Distribution techniques
5.1 Do not restrict access through digital rights management
EPUB publications typically require preservation of the
publisher's and author's intellectual property when distributed (e.g., so that they can be made
available for individual sale through online bookstores or distributed through library systems). The
most common way to address this need has been through the application of digital rights management
(DRM) schemes to the packaged EPUB publication. DRM enables a variety of security features that
aren't native to the EPUB format, such as the ability to limit access to a single user and to limit
the length of time the person can access the publication (e.g., library loans).
In general, DRM can be made to work interoperably with assistive technologies, but problems arise
when DRM restrictions remove direct access to an EPUB publication or restrict access to the content
within it. Unless the reading system implementing
the DRM provides API level access to the content, it can prove difficult, or even impossible, to
generate text-to-speech playback, or for a refreshable braille display to have access to the
underlying text, as well as cause other accessibility issues.
Therefore, the application of digital rights management cannot impair or impede the functionality of
assistive technologies on EPUB publications users have the right to access.
5.2 Include accessibility metadata in distribution records
When an EPUB publication is ingested into a
distribution system, such as a bookstore or library, a metadata record is often provided separately
to the distributor. In these scenarios, the metadata used to enable discovery of the publication
typically comes from the distribution record alone, not from the metadata in the package
document.
The result is that it is necessary to include as much accessibility metadata in distribution records
as their vocabularies allow.
Note
The use of distribution records does not remove the requirement to include accessibility metadata
in the package document. The metadata in the
package document ensures accessibility information is always available with the publication.
5.2.1 Helpful resources
See the following resources for more information about including accessibility metadata in
distribution records: