The Understanding EPUB Accessibility guide provides information on how to evaluate the accessible content conformance requirements of [[[epub-a11y-12]]] [[epub-a11y-12]] against reflowable EPUB publications.
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This document is limited in focus to reflowable EPUB publications [[epub-3]].
It does not provide techniques for meeting the objectives of [[[epub-a11y-12]]. Refer to [[[epub-a11y-tech-12]]] for more information about how to meet specific success criteria and objectives.
Although, in many cases, the guidance will also prove useful for evaluating fixed-layout EPUB publications [[epub-3]], making those kinds of publications accessible presents unique challenges. Guidance on making fixed layouts more accessible is instead provided in [[[epub-fxl-a11y]]] [[epub-fxl-a11y]].
The [[wcag2]] accessible authentication success criteria — both the minimum (3.3.8) and enhanced (3.3.9) …
Web sites are constructed very differently from EPUB publications. A typical web site wraps the content of each page within a repeating template, for example. This template gives each page a consistent look and feel, but users are rarely interested in the wrapper content after visiting the first page. Visual readers can typically skip past the site header, navigation bars, search boxes, and other helpful but seldom-used features to get right to the content.
To provide the same ease of access to readers who would have to navigate sequentially through the repetitive content, the [[wcag2]] bypass blocks success criterion (2.4.1) requires a means of bypassing the repeated content in a set of pages. This success criterion does not apply to typical EPUB publications, however, as EPUB content documents do not repeat content in the same way that web sites do.
Each new content document might begin with similar content, such as learning objectives or key terms, but this content is part of the body of the publication and not identical to what came before. Consequently, it is not required to add a link to skip it. (Secondary content still needs to be identified in accordance with success criterion 1.3.1, however.)
If an EPUB publication were to reproduce a set of web pages with their full site trappings, then success criterion 2.4.1 would apply, but this practice is not common.
The [[wcag2]] consistent help success criterion (3.2.6) …
The [[wcag2]] meaningful sequence success criterion (1.3.2) specifies that each web page have a meaningful order (i.e., that the visual presentation of the content match the underlying markup).
As EPUB allows two EPUB content documents to be rendered together in a synthetic spread [[epub-3]], the order of content within a single document cannot always be evaluated in isolation. Content might span visually from one document to the next. For example, a sidebar might span the bottom of two pages.
Ordering each document separately by the visual display will lead to users of assistive technologies encountering gaps between the start and end of the spanned text. If the markup cannot be arranged to provide a more logical reading experience (e.g., the beginning of the spanned content at the end of the first page followed by the conclusion at the start of the next), another means of satisfying this criteria will be necessary to avoid failure (e.g., a hyperlink could be provided to allow a user to jump from the break point on the first page to the continuation on the next).
The [[wcag2]] multiple ways success criterion requires that web users are able to locate content within a set of pages on a site. This ensures that there are at least two ways to reach any information.
This requirement is equally helpful for reading digital publications like EPUB because without multiple ways to reach into a publication a user would have to manually swipe or click from page to page to read. If this were the case, readers of print books would have better access, as they could read page to page or flip through the pages using the page numbers to locate a passage they are looking for.
Fortunately, EPUB publications, unlike web site, will typically always minimally meet this success criterion both because of the way they are constructed and because of common affordances offered by reading systems.
From an authoring perspective, the primary requirements are to list all all EPUB content documents in the spine and to ensure that users have a secondary means of accessing all non-linear documents [[epub-3]]. This ensures that users have a means of accessing all the content either by paging through the publication, for linear content, or following hyperlinks to reach any non-linear content a reading system might not show in the default presentation.
EPUB's requirement for a table of contents provides the second method of access. It is possible that a publication could fail this success criterion by proividing an incomplete table of contents, but such occurrences are rare. (Refer to the note on the completeness of the table of contents for further discussion.)
While these two basic requirements of EPUB satisfy the need for multiple ways to access the content on their own, publications often contain additional methods of access, such as:
Reading systems also facilitate access into the content through text search capability and also via the ability to jump to dynamically generated pages.
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 is to create a simple aggregation of all the headings for all the sections, practically there are several 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 headings below a certain depth get trimmed to improve the readability. Further, reading systems do not always provide structured access to the headings in the table of contents, or provide shortcuts to navigate the links. The result is that users have to listen to each link one at a time to find where they want to go, a tedious and time-consuming process.
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 opting not to provide links to all the headings, it is best to optimize the links that are provided to improve the 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; and
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).
The [[wcag2]] redundant entry success criterion (3.3.7) …
The [[wcag2]] reflow success criterion (1.4.10) seeks to ensure that content remains readable for users with low vision as they enlarge it. It requires that there be no loss of information or readability for vertically scrolling content at a width equivalent to 320 CSS pixels [[wcag2]].
As EPUB reading systems do not support horizontally scrolling text, and such content is rarely produced even on the web generally, the requirement for horizontally scrolling content at a height of 256 CSS pixels is not discussed in this section.
The first thing to note about this success criterion is that the "scrolling" aspect of the definition applies to all EPUB publications whether they are read in a reading system that lays out the content in a scrolling manner or whether the reading system dynamically paginates the content. Dynamic pagination is just a feature of EPUB that takes a vertically scrolled document and splits it up for reading in a book-like manner.
The second thing to note is that the width and height sizes do not refer to the actual screen resolution of the device. The emphasis of the success criterion is on equivalence up to these sizes, which is why it notes that a viewport with a width of 1280 CSS pixels is equivalent to a width of 320 CSS pixels when it is zoomed to 400%. The example is just a common screen resolution and exhibits easy math to the 320 CSS pixel maximum.
What happens when zooming a 1280 CSS pixel viewport beyond 320 CSS pixels is out of scope, as is zooming any other starting resolution after it exceeds 320 CSS pixels in width. The success criterion is not saying that all viewports greater in width than 320 CSS pixels are out of scope.
The good thing about reflowable EPUB publications when it comes to meeting this success criterion is that they were designed to work exactly in the manner it anticipates. A reflowable EPUB publication is expected to work on many different device screens and with many different user preferences applied. The content has to reflow to fit within the paginated environment that reading systems provide as users rarely can scroll the content — if the content does not fit in the viewport, it is typically inaccessible to all users. Publishers are typically already aware of these reflow issues and how to ensure their content does not break.
It does not mean that all reflowable EPUB publications get an automatic pass on this success criterion, but only that the circumstances where the content will not reflow are not common.
The most likely source of failure for the reflow success criterion is going to arise with container elements, such as with the [[html]] [^aside^], [^div^], [^nav^] and similar elements used to group content. When these elements are assigned fixed widths, their ability to adapt to changes in zoom diminish.
The content of an [[html]] [^aside^] element used as a sidebar, for example, will reflow by
default to so that its content fits the available screen space. But if a width, or a minimum
width, greater than 320 CSS pixels is assigned, then as the content is zoomed the dimensions of
the viewport may become smaller than the width of the aside.
When a user zooms their publication, what typically will happen with EPUB reading systems is that the sidebar will extend outside the boundary of the page/viewport, causing its text to be cut off and become inaccessible.
But even if the sidebar is made scrollable using CSS overflow properties [[css-overflow]], the result still fails this success criterion because the content now requires scrolling on the x axis to reach the hidden overflow text.
Any hoped-for value in assigning fixed widths to container elements in reflowable content is
generally going to be offset by the problems it will cause with the reflow success criterion. If
a size must be specified (e.g., to ensure that boxes of content always fill the width of the
viewport), the more accessible way is to use a unit that adapts to changes in the viewport.
Setting a percentage (%) or viewport width (vw) no greater than 100
percent of the viewport size will avoid scroll bars appearing as the content is zoomed.
Container elements can require vertical scrolling. The reflow success criterion only requires that the content not require scrolling on the opposite axis.
When testing this success criterion, be aware that app-based reading systems typically do not allow zooming of reflowable content. They are more likely to support font size increases up to 400% of the default, but increasing the font size is not the same as zooming the content.
The reflow success criterion does not, in fact, specify requirements on the font size as the content is enlarged. The requirement for text to enlarge up to 200% is instead addressed by the [[wcag2]] resize text success criterion.
For a more detailed description of the interaction between the reflow and resize text success criteria, refer to the reflow understanding document explanation of the two.
Large words, hyperlinks, code examples, and other strings of contiguous text with no break points can also pose problems for reflow as the viewport reaches 320 CSS pixels, but the text is not required to stay the same font size as the content is enlarged. The font size could, for example, be reduced using a media query so that it stays at 200% the original size as the viewport nears 320 CSS pixels.
Other strategies to prevent large text strings causing scrolling include using the [[html]]
[^wbr^] element to specify word break locations, using the CSS overflow-wrap and
word-break properties, or using the Unicode soft hyphen character.
Not all content is required to reflow as it is enlarged. The success criterion makes an exception for content that would lose its meaning if it were reflowed. For reflowable publications, the most common content that requires two-dimensional layout are diagrams, charts, maps and similar images.
Data tables are also exempt due to their grid nature, but the individual cells within them are not. This means that it would not be unreasonable for the user to have to scroll to reach all the columns, but the content with a column must not require scrolling in two dimensions to read.
Content that can be scrolled in two dimensions needs to always be checked that it has been
authored so that it can indeed be scrolled this way. Reading systems will often try to compress
tables to fit the available space, for example, but when that cannot be done the overflow
outside the page boundary will not be accessible. A common practice to avoid this is to wrap the
two-dimensional content in a container that allows scrolling (e.g., a table can be
placed inside a div that provides it more space for rendering and includes
scrollbars to move around the columns and rows). But when using this practice, content that does
not depend on two dimensions for comprehension cannot be captured in the scroll. If a table has
a caption, for example, only the table can scroll on the x axis to meet the success criterion;
the text of the caption cannot require scrolling.
Although the reflow success criterion does not pose a lot of problems for reflowable EPUB publications, it is one of the most problematic to meet for fixed-layout EPUB publications as reading systems do not reflow the content of a fixed-layout page as it is zoomed.
This guide does not address the problems with making fixed-layout publication accessible. For more information on this topic, refer to [[[epub-fxl-a11y]]] [[epub-fxl-a11y]].
Although it is possible for users who require a publication in audio form to use text-to-speech playback, the experience is considerably poorer than when pre-recorded narration is provided. Text-to-speech engines have limited built-in vocabularies, causing them to mangle and mispronounce most uncommon words they encounter. As a result users have to have words repeated and spelled out to make sense of the content, slowing down their reading and reducing comprehension.
For this reason, it is important to provide narration for the full text of a publication in addition to the full text. Users can then decide which reading modality they prefer — text, audio, or a mix of the two.
When reading visually, users can quickly move through, and escape from, highly structured content such as sidebars, lists, and figures. Visual readers can skim lists and quickly return to the primary narrative once they locate the desired information, for example. The same is true for reading figures and sidebars, as they are visually offset from the primary narrative so easily jumped into and out of.
The same ease of escaping from content is only possible if EPUB publications include structural semantics in the synchronized text/audio format. Users might not be able to escape from lists, sidebars, figures, and other highly structured content, without the structural semantics of those elements being available.
When the synchronized text-audio playback instructions include this information, reading systems can simplify playback for auditory readers to enable a comparable reading experience.
Every EPUB publication has a default reading order that allows users to progress through the content. The default reading order consists of two parts: the order of references in the spine provides a high-level progression through the EPUB content documents that make up the publication, while the markup within each EPUB content document provides the default progression through the content elements (i.e., as represented in the document object model [[?dom]]).
For many languages, the default reading order also matches the logical reading order — the way that users will naturally follow the narrative. It ensures that readers can follow the primary narrative and that they encounter secondary content where the author intended it to be read. The default reading order also establishes some less obvious relations, like the progress within a table from cell to cell and row to row.
If the sequence of the synchronized text-audio playback does not match this progression, it can cause confusion for readers, whether they are only listening to the audio or trying to also follow visually.
Ordering the playback to match the default reading order is the safest way to ensure that users can follow the text. In some cases, however, strict adherence to this practice can result in a suboptimal reading experience (e.g., playback of a table by column instead of row might make more natural sense in some cases). These publications have a logical reading order that cannot match the default order of the elements of the host format.
The goal of this objective is not to forbid alternate presentations, but to ensure that deviations are only made to better represent the logical reading order of the content.
Being able to read the primary narrative of a work without interruption is central to reading comprehension. EPUB publications are typically structured to visually represent secondary information such as page break markers and footnotes outside the main narrative flow (e.g., by using different background colors or placement so readers can filter this information visually out while reading).
Readers who prefer auditory playback, however, cannot skip this information with the same ease in a linear audio-based reading experience. And, without structural semantics, synchronized text-audio playback cannot offer skipping content either.
When the synchronized text-audio playback instructions include structural semantics, however, reading systems can create reading experiences that allow users to decide which secondary content to skip by default.
If a success criterion does not apply to an EPUB publication, it should never be reported as an unqualified pass. Doing so would give the false impression that the publication contains the inapplicable content and that it was found to be conforming.
When reporting success criteria that do not apply to a publication, it is recommended to label their status as "not applicable" (or an equivalent language-specific term). If a "pass" value must be assigned by the reporting template used, then a comment should be included that clearly states that the publication contains no applicable content.