Transcript - Introduction to accessible EPUBs - Accessible publishing

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Introduction to accessible EPUBs

[Narrator:] Welcome. In this chapter, you will learn how to create accessible e-books using the EPUB data format. EPUB is the abbreviation for electronic publication. It is a distribution and interchange format standard for digital publications and documents. EPUB was formerly defined by the International Digital Publishing Forum, which is now part of the World Wide Web Consortium. Together they form the EPUB 3 Community Group. As of version 3, EPUB is also an ISO standard. Its identification is ISO/IEC TS 30135.

An EPUB is a bundled collection of resources that can be transferred to an EPUB reader to make its content available to a user. A reader can be a hardware device in the form of an e-book reader, an app on a mobile device or a software application on a PC. The resources of an EPUB typically consist of web content, including HTML, CSS, SVG, images, fonts and other resources. These files are bundled for distribution in a ZIP-based archive with the file extension .epub. Using the common ZIP compression format, publications can be unzipped by many software programs, simplifying both their production and consumption.

Please note that one of the major players in the e-book market, Amazon, with its Kindle platform, does not support the EPUB file format. The Amazon Kindle uses proprietary closed file formats. There are free software solutions available that can convert unprotected EPUB files, without using any Digital Rights Management, into the Kindle file format.

Key features of EPUB accessibility

The EPUB standard defines several accessibility features.

  • Navigation: Every EPUB document should support a flexible navigation system. The need to navigate a document is not exclusively an accessibility issue, but complete navigation is key for all users to be able to easily access the contents of a document.
  • Semantic markup: HTML5 file supports a number of new elements intended to make markup more semantically meaningful; for example: header, footer, nav, aside and ARIA roles. These elements, in conjunction with best practices for authoring web content, should be used when creating EPUB documents.
  • Dynamic layouts: EPUB is designed for reflowable content, meaning that an EPUB reader can optimise text for a particular resolution-independent display device. Users prefer to decide themselves at which text size or contrast the content should be rendered.
  • Aural renditions and media overlays: EPUB 3 provides text-to-speech facilities for controlling aspects of speech synthesis, such as pronunciation, prosody and voice characteristics. Of course, the e-book reader has to support this technology.
  • Fallbacks: Not all formats are accessible in their native format, and not all users prefer to read in the default format provided. EPUB defines a variety of means for providing fallbacks so that alternate renditions of a publication can be made available in these cases. This could be useful when rendering audio information to users with hearing disabilities or video information to users with visual disabilities.

EPUB reader hardware

An e-book reader is a portable electronic device that is designed primarily for the purpose of reading e-books and periodicals. It has a similar form to a tablet and usually refers to a device that uses an e-ink display resulting in better screen readability, especially in bright sunlight and longer battery life when compared to a tablet.

E-ink displays differ in their size, resolution, contrast ratio and the number of greyscale levels they can display. Some offer a backlight to allow for low-light reading.

There are many manufacturers producing e-book readers capable of reading the EPUB format. As this course does not promote specific brands, we have not listed them here. Not every e-book reader can read the EPUB file format. The most popular exception is Amazon with its Kindle.

E-book readers often offer support for multiple document formats, for example PDF.

Most e-book readers support several accessibility functions. They are able to: change the size of the text, change the style and font of the text, increase contrast or brightness, and invert text and background colour. Beyond that, some e-book readers offer a text-to-speech functionality and some even offer a real screen reader. Some e-book readers address users with visual disabilities and most of them are able to handle DAISY and EPUB e-books.

The EPUB standard allows the integration of media content, however not every e-book reader can make use of it. If you want to make use of media content, be sure to check if your e-book reader supports the required features. If your e-book reader has no speakers and no headphone jack, then it will not be able to render audio output.

E-ink displays are optimised for displaying the same static page content for a long time. Typically, e-ink displays only consume power when updating the display. They are therefore a poor choice when it comes to rendering videos or animations.

EPUB reader software

Even though e-book readers offer fantastic possibilities to read e-books, users often prefer to use their existing devices. There is a rich choice of EPUB-compatible e-book readers on nearly every smartphone, tablet or PC. Here is a small selection of free available software: Apple’s iBook software on iOS and macOS; Google Play Books on Android; and Adobe Digital Editions for Apple’s iOS and macOS, Microsoft Windows and Android.

There are hundreds of software e-book readers. Some of them are available for free, others are commercial. Some of them come as stand-alone software, others as a web browser extension. We selected the ones listed because they promote their usage with assistive technologies like screen readers on all available platforms.>

As smartphones, tablets and PCs are capable of rendering media content, many of these readers can render audio and video embedded inside an EPUB.>


As a special tool, Calibre deserves to be mentioned. Calibre is a cross-platform open-source e-book manager. It can organise existing e-books into virtual libraries and display, edit, create and convert e-books, as well as syncing e-books with a variety of e-readers. It is available for free on Windows, macOS and Linux.

Calibre is capable of converting unprotected e-books between different file formats. It can be used to convert an EPUB to one of Amazon Kindle’s proprietary file formats. We will have now a closer look at the EPUB file format. Please note that this course cannot explain every detail of the EPUB file format. Our aim is more to develop a basic understanding of what EPUB is and how it works. For details, please refer to the EPUB standard, for which you will find the link under the Documents tab on our platform.

As previously mentioned, an EPUB file uses the ZIP compression file format to bundle its resources. Here we have an EPUB file. We copy it and change the file suffix from .epub to .zip. Uncompressing the file creates a new folder offering access to the bundled resources. We can see one file and two folders.

Let’s look at the file named mimetype. When we open the file in a text editor, we see the media type. EPUB readers use the mimetype file to check if the document really contains EPUB data. A reader that cannot find this file using this media type will immediately abort file handling.

The folder META-INF, short for meta information, contains the required file container.xml. There might be more files in this directory providing additional information for different readers. We can open the container file using a text editor. The XML data points to the OPF file, traditionally named content.opf, defining the contents of the book. OPF stands for Open Package Format. The OPF file can be opened with a text editor. The XML-based OPF file structure shows a package root element with four child elements: metadata, manifest, spine and guide. The metadata block offers information about the language, the title, the author, the publisher, the identifier and much more. The manifest block references all the files contained in the package. Each file is represented by an item element, and has the attributes id, href and media-type. A file in the package which does not appear in the manifest cannot be accessed by an e-book reader. As you can see here, we find references to all embedded HTML files, style sheets, the table of contents, fonts and images. If the EPUB offered more file types, like audio or video information, then those files would appear here too.

As the spine of a hardcover book binds all the paper pages, the spine block collects all HTML documents in their linear reading order. Any content document that can be reached through linking or the table of contents must be listed as well. Note how the spine references the HTML files by their ID.

The optional guide block identifies other structural components of the book. Each reference element has the attributes type, title and href. Files referenced in href must be listed in the manifest and are allowed to have an element identifier. Guides can be used to mark the cover page or to reference different indexes of the document; for example, a list of illustrations.

Two ways to create a table of contents

EPUB files following a standard version below 3 use an NCX file for creating the table of contents. NCX is short for Navigation Control file for XML. Publications using EPUB versions 3 and above use an EPUB Navigation Document. We will have a short look at both of them. As a matter of fact, it is possible to use both approaches in the same publication to increase compatibility with a maximum number of readers.

Table of contents using an NCX file

For referencing an NCX, the spine uses the toc attribute, which contains the id of the NCX file listed in the manifest. The hierarchical table of contents of an EPUB file is traditionally named toc.ncx. Please note that even though NCX is widely used, it is a legacy file format. The head block contains metadata items. The dtb:uid value should be identical to the identifier metadata in the OPF file. The dtb:depth value indicates the maximum depth of the navMap element, which we will introduce in a few seconds. It tells readers how far down the navigation tree should go when building the table of contents. When there is no nesting, the maximum depth is 0. You may wonder what dtb stands for in these identifiers. dtb stands for Digital Talking Books.


The specification for NCX was developed for DAISY Digital Talking Books. DAISY, short for Digital Accessible Information System, was designed to be a complete audio substitute for print material and is specifically designed for use by people with print disabilities, including blindness, impaired vision and dyslexia.

The DAISY consortium has been involved in the standardisation of EPUB since its inception at the International Digital Publishing Forum and within the World Wide Web Consortium.

The title in the docTitle block should be identical to the title set in the OPF file. The navMap block is a wrapper element for the element data tree used to generate the table of contents. Each navPoint element inside the navMap generates one entry in the table of contents.

NavPoint elements can be nested to create navigation points to major subheadings inside each chapter of the book. NavPoint elements have two key attributes: id, which needs to be unique for each navPoint in the NCX, and playOrder, which is used to put all of the navPoint elements in the correct reading order. The navLabel inside a navPoint hosts the text label of a chapter.

If your EPUB has a print counterpart, it is a good idea to include a pageList in the EPUB. A pageList maps your e-book content to print pages. This helps readers who may be referring to both the print and electronic versions of the content; for example, in a classroom setting or when looking up a citation.

EPUB Navigation Document

Publications using EPUB versions 3 and above use an EPUB Navigation Document. The EPUB Navigation Document is an XHTML Content Document, but with additional restrictions on its structure to facilitate the machine-processing of its contents.

The OPF file shows that we can find our table of contents in the file toc.xhtml. We have opened the file and can see an XHTML file presenting a web page. Please note that this web page has additional required elements and attributes that make it recognisable as an EPUB Navigation Document.

Here is the EPUB specific namespace. The section using the attribute epub:type should be placed directly inside the body element. A special nav using the attribute epub:type with value toc indicates the start of the table of contents. An ordered list presents all the different chapters and sub-chapters. An EPUB Navigation Document may offer additional guides, like a list of illustrations, in the form of an additional nav element using the epub:type value of landmarks.


So far we have seen a lot of meta information. Let’s look inside a content page. A content page looks pretty much like a normal web page. The pages can be styled like any other web page. Please note that some e-book readers may not be able to render all the types of styles you may choose.

EPUB accessibility

As the content of an EPUB is based on web technologies, the Web Content Accessibility guidelines, which are commonly called the WCAG, can be used as a criterion for improving the accessibility of EPUB files. At the time of the publication, the WCAG were available in version 2.1. The WCAG, however, focus on the accessibility of single web pages, whereas an EPUB is a container of pages. Therefore, additional criteria have been defined to improve the accessibility of a complete package.

These criteria are defined in the EPUB Accessibility – Conformance and Discovery Requirements for EPUB Publications specification. This specification addresses two key needs in the EPUB ecosystem: the evaluation and certification of accessible EPUBs, and the discovery of the accessible qualities of EPUBs.

The specification defines three categories of compliance for EPUBs: A discoverable document offers accessibility metadata, which enables users to detect whether an EPUB is appropriate for their needs or not. It does this regardless of its grade of accessibility. An EPUB document stating that it is not accessible for a specific target group can still be classed as discoverable. An accessible document meets all discovery, WCAG 2 and EPUB accessibility requirements. An optimised document meets the discovery requirements optimised for a specific target group. We will have a look at all three categories.

Discoverable category

Unlike web pages, EPUBs are designed to be distributed through many channels for personal use. A consequence of this model is that specific details about the accessibility of a publication need to travel with it. An online bookshop receiving content from different sources does not know the production quality that has gone into each submission, so can only convey to consumers what is present in each publication’s metadata. A user with specific needs wants to make sure that the document in question meets the required accessibility criteria before obtaining it. A user can only decide if the content is suitable for them through the inclusion of rich metadata.

Every discoverable EPUB must include the following accessibility metadata.

The accessMode metadata: Here are the most commonly used values for the accessMode metadata that are typically specified for EPUBs:

  • Textual: the publication contains text content such as headings, paragraphs, etc.
  • Visual: the publication contains visual content such as images, graphics, diagrams, animations and video.
  • Auditory: the publication contains auditory content such as standalone audio clips and audio soundtracks for video content.

These lines show the metadata entries for an EPUB 3 publication that has textual and visual access modes. The third line indicates that the content can be consumed with a textual and visual access mode. As the author of the document has enhanced the images with alternative descriptions, a fourth line can be added stating that all content can be consumed using a textual mode only.

The accessibilityFeature metadata: Identifying all the accessibility features and adaptations included in an EPUB allows users to determine whether the content can be used at a more fine-grained level than the access modes do. These lines indicate that the document uses MathML for coding mathematical formulae and offers alternative texts.

The accessibilityHazard metadata: Authors have to report whether or not their EPUBscontain resources that present any hazards to users, as they can have real physical effects. This line warns the user of flashing content. If a resource flashes more than three times a second, for example in videos and animations, it can cause seizures.

The accessibilitySummary metadata: An accessibility summary provides a brief, human-readable description of the accessibility characteristics of an EPUB, or lack thereof. This mandatory metadata information must be included for the EPUB to be compliant. The standard defines the possible values for accessMode, accessibilityFeature and accessibilityHazard metadata.

For additional metadata information and a list of possible values, please refer to the specification.

Accessible category

An accessible EPUB document must meet the following criteria to be accessible. It must include discovery metadata. It must meet the requirements for WCAG 2 conformance. As previously seen, EPUB is built on web technologies. The primary source for the production of accessible web content is the Web Content Accessibility Guidelines, or WCAG for short. The EPUB Accessibility specification refers to the WCAG standard. For more information about the WCAG standard, please refer to the chapter Introduction to web standards. It must provide navigation to static page break locations.

Printed books with statically paginated content continue to be the most consumed medium for books, both among the general reading public and in educational settings. As a result, a non-visual reader in an environment where statically paginated content is used is disadvantaged by not being able to easily address the same locations in the publication; for example, a teacher instructing students to turn to a specific page.

Here is an example of how to specify the beginning of the first page. It must include accessibility conformance metadata. To indicate that an EPUB conforms to the accessibility requirements, it must include a conformsTo property in the metadata section of the OPF. The extensions wcag-a, wcag-aa and wcag-aaa indicate WCAG levels of compliance A, AA and AAA.

The a11y:certifiedBy property specifies the name of the party that certified the content. The certifier of the content can be the same party that created the EPUB, but can also be a third-party accessibility certifier. If the party that certifies the content has provided a detailed report of its assessment, a link to the assessment can be provided in an a11y:certifierReport property. Here is an example of how this could be expressed in a metadata section.

Optimised category

Not every document targets the general public. For example, an EPUB with synchronised text and audio might contain a full audio recording of the content but limit the text content to only the major headings. In this case, the document is consumable by users who can hear the content, but it is not usable by users with auditory disabilities. In other words, when an EPUB is optimised for specific reading modalities, the failure to achieve a WCAG conformance level does not make it any less accessible to the intended audience.

An optimised EPUB must meet the following criteria. It must include discovery metadata. It must identify the standard or guidelines the content adheres to in a conformsTo property. The value of this property must be an Internationalised Resource Identifier that references the standard or guidelines it follows. If the resource identifier is not sufficient for a user to understand conformance, for example if the guidelines are not publicly available, more information about how the content has been optimised should be provided in the accessibility summary.


The creation of an accessible EPUB does not in itself guarantee that the content will be obtainable by users. Depending on the distribution system used, other factors will influence the overall accessibility of the EPUB. If the EPUB files are distributed via an online bookshop, then the website distributing the EPUB files needs to be accessible too. A conformant distribution system must meet the following criteria. Its user interface must conform to WCAG level AA. It must provide the ability to narrow search results by the available accessibility metadata.

What this chapter did not cover

We have introduced many features of the EPUB file format, but we have not discussed them all. Here are the features we did not cover.

While we have discussed the EPUB format, we have mostly neglected the media capabilities of this file format. The EPUB 3 specification defines the possibility of Media Overlays. Synchronised audio is found in mainstream e-books, educational tools and e-books formatted for persons with print disabilities. In EPUB 3, audio can be embedded using Media Overlay documents to describe the timing of the pre-recorded audio and how it relates to the EPUB content document markup. The file format for Media Overlays is defined as a subset of SMIL, short for Synchronized Multimedia Integration Language, a W3C recommendation for representing synchronised multimedia information in XML. The Media Overlay feature is designed to be transparent to EPUB reading systems that do not support this feature. The inclusion of Media Overlays in an EPUB has no impact on the ability of Media Overlay-unaware reading systems to render the EPUB as though the Media Overlays were not present. As required by the WCAG, there should always be text alternatives for audio content.

An EPUB file can optionally contain digital rights management as an additional layer, but this is not required by the specification. Digital rights management describes access control technologies that are used by hardware manufacturers, publishers, copyright holders and individuals with the intent to limit the use of digital content and devices after sale. Typically digital rights management is used for commercial content. As the application of digital rights management enforces a strong connection between authoring, distribution and the reading platform, we will not discuss its consequences on accessibility here.

Another set of features we have not mentioned is text-to-speech facilities. EPUB 3 provides text-to-speech facilities for controlling aspects of speech synthesis, such as pronunciation, prosody and voice characteristics. As we are not aware of any available e-book reader supporting these features, we will not discuss them here.

Where to continue?

You have seen an introduction to EPUB and its accessibility features. You have seen how to improve the accessibility of EPUB documents at a code level.

Depending on your personal interests, you could continue with one of the following chapters:

  • EPUB tools

  • EPUB testing

  • From Adobe InDesign to EPUB

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