EPUB 3 Overview

Every EPUB publication is represented by a package document. The package document specifies all the resources required to render that representation of the content. The package document also defines a reading order for linear consumption, and associates metadata and navigation information.

The package document defines a layer on top of the traditional structuring of a typical web site to facilitate the authoring of digital publications. A web site, for example, embeds references to its resources within its content, which, while a simple and flexible means of identifying resources, makes it difficult to enumerate all the resources required to render it. In addition, there is no standard way for a web site to define that a sequence of pages make up a larger publication, which is precisely what EPUB's spine element [epub-33] does (i.e., it provides an external declarative means to explicitly specify navigation through a collection of documents). Finally, the package document defines a standard way to represent metadata globally applicable to a collection of pages.

The package document also includes a collection element [epub-33], which allows grouping of logically related publication resources. This element exists to enable the development of specialized content identification, processing, and rendering features such as the ability to define embedded preview content or assemble an index or dictionary from its constituent XHTML content documents.

The package document is specified in the dedicated section of [epub-33].

EPUB publications provide a rich array of options for adding metadata. Each package document includes a dedicated metadata section [epub-33] for general information about the EPUB publication, allowing titles, authors, identifiers, and other information about the EPUB publication to be easily accessed. It also provides the means to attach complete bibliographic records using the link element [epub-33].

The package document also allows a Unique Identifier to be established for the EPUB publication using the unique-identifier attribute [epub-33].

XHTML content documents also include the means of annotating document markup with rich metadata, making them more semantically meaningful and useful both for processing and accessibility purposes. Both RDFa [rdfa-in-html] and Microdata [microdata] attributes can be used in XHTML content documents for that purpose.

Each EPUB publication contains one or more EPUB content documents, as defined in the dedicated section of [epub-33]. These are XHTML or SVG documents that describe the readable content and reference associated media resources (e.g., images, audio, and video clips).

XHTML content documents are defined by a profile of [html].

Although EPUB's history is steeped in enabling reflowable content, not all publications lend themselves easily to reflowing. Page-precise layouts are required to meaningfully represent children's books, comics and manga, magazines, and many other content forms.

EPUB 3 includes metadata that allows the creation of fixed-layout XHTML content documents [epub-33], in addition to existing capabilities for fixed layouts in SVG. This metadata enables the control of the page dimensions [epub-33], creating a canvas on which elements can be absolutely positioned.

The metadata does not just flag whether content is to be fixed or reflowed, but also allows EPUB creators to specify the desired orientation of pages [epub-33], when to create synthetic spreads [epub-33], and how to position pages [epub-33] within those spreads, providing a broad range of control over the presentation of EPUB publications.

The structure and semantics of EPUB Documents with Fixed Layouts are defined in [epub-33].

A key concept of EPUB is that content presentation adapts to the user, rather than the user having to adapt to a particular presentation of content. HTML was originally designed to support dynamic rendering of structured content, but over time HTML as supported in web browsers has become focused on the needs of web applications, and many popular web sites now have layouts with less flexibility.

EPUB publications, however, are designed to maximize accessibility for the visually impaired, and reading systems typically perform text line layout and pagination on the fly, adapting to the size of the display area, the user's preferred font size, and other environmental factors. This behavior is not guaranteed in EPUB; images, vector graphics, video, and other non-reflowable content might be included, and some reading systems might not paginate on the fly, or at all. Nevertheless, supporting dynamic adaptive layout and accessibility has been a primary design consideration throughout the evolution of the EPUB standard.

EPUB content documents can reference CSS Style Sheets, allowing EPUB creators to define the desired rendering properties. EPUB 3 follows support for CSS as defined in the [csssnapshot].

EPUB 3 also supports CSS styles that enable both horizontal and vertical layout and both left-to-right and right-to-left writing.

EPUB 3 supports audio and video embedded in XHTML content documents via the [html] audio and video elements, inheriting all the functionality and features these elements provide. For more information on audio and video formats, refer to the section on core media types [epub-33].

Another key multimedia feature in EPUB 3 is the inclusion of media overlay documents [epub-33]. When pre-recorded narration is available for an EPUB publication, media overlays provide the ability to synchronize that audio with the text of a Content Document (see also 4.4 Aural renditions and media overlays).

The structure and semantics of media overlay documents are defined in [epub-33].

EPUB 3 supports two closely related font formats — OpenType [opentype] and WOFF [woff] [woff2] — to accommodate both traditional publishing workflows and emerging web-based workflows. Word processing programs used to create EPUB publications are likely to have access only to a collection of installed OpenType fonts, for example, whereas web-archival EPUB generators will likely only have access to WOFF resources (which cannot be converted to OpenType without undesirable, and potentially unlicensed, stripping of WOFF metadata).

EPUB 3 also supports both obfuscated and regular font resources for both OpenType and WOFF font formats. Support for obfuscated font resources is required to accommodate font licensing restrictions for many commercially available fonts. See the dedicated section on Font Obfuscation in [epub-33] for more details.

EPUB strives to treat content declaratively — as data that can be manipulated, not as programs to be executed — but does support scripting as defined in [html] and [svg] (refer to the section on Scripting [epub-33] for more information).

It is important to note, however, that EPUB 3 does not require scripting support in reading systems, and scripting might be disabled for security reasons.

EPUB creators need to be aware that scripting in an EPUB publication can create security considerations that are different from scripting within a web browser. For example, typical same-origin policies are not applicable to content that has been downloaded to a user's local system. Therefore, it is strongly encouraged that scripting be limited to container constrained contexts, as further described in the section on container-constrained scripts [epub-33].

In other words, consider limiting scripting to cases where it is essential to the user experience, since it greatly increases the likelihood that content will not be portable across all reading systems and creates barriers to accessibility and content reusability.

EPUB 3 provides the following text-to-speech (TTS) facilities for controlling aspects of speech synthesis, such as pronunciation, prosody, and voice characteristics:

Pronunciation Lexicons

The inclusion of generic pronunciation lexicons using the W3C PLS format [pronunciation-lexicon] enables EPUB creators to provide pronunciation rules that apply to the entire EPUB publication. Refer to Pronunciation Lexicons [epub-tts-10] for more information.

Inline SSML Phonemes

The incorporation of SSML phonemes functionality [ssml] directly into a EPUB content document enables fine-grained pronunciation control, taking precedence over default pronunciation rules and/or referenced pronunciation lexicons (as provided by the PLS format mentioned above). Refer to SSML Attributes [epub-tts-10] for more information.

Note that EPUB creators may also rely on CSS Speech [css-speech-1] properties in their style sheet definitions.

The authoring features for improving the voicing of EPUB 3 publication are described in the separate Working Group Note on Text-to-Speech Enhancements [epub-tts-10].

An EPUB publication is transported and interchanged as a single file (a "portable document") that contains the package documents, all EPUB content documents, and all other required resources for processing the Publication. The single-file container format for EPUB is based on the widely adopted ZIP format, and an XML document that identifies the location of the package document for the Publication in the ZIP archive is located at a pre-defined location within the archive.

This approach provides a clear contract between any creator of an EPUB publication and any system which consumes such EPUB publications, as well as a reliable representation that is independent of network transport or file system specifics.

An EPUB publication's representation as a container file is specified in the dedicated section of [epub-33].

EPUB 3 supports alternate representations of all text metadata items in the package metadata section to improve global distribution of EPUB publications. The alternate-script property [epub-33] can be combined with the xml:lang attribute to include and identify alternate script renderings of language-specific metadata.

Using this property, a Japanese EPUB publication could, for example, include an alternate Roman-script representation of the author's name and/or one or more representations of the title in a Romance language.

Text metadata items may also rely on attributes specifying base direction [epub-33] that allow for a finer control of bidirectional texts (e.g., titles that combine Latin and Arabic or Hebrew characters).

Finally, the page-progression-direction attribute allows the content flow direction to be globally specified for all EPUB content documents to facilitate rendering (see the page-progression-direction [epub-33]).

EPUB 3's support for PLS documents and SSML attributes increases the pronunciation control that EPUB creators have over the rendering of any natural language in text-to-speech-enabled reading systems. Refer to 2.10 Text-to-speech in the Features section for more information on these capabilities.

The OCF container format supports UTF-8, allowing for internationalized file and directory naming of content resources.

As noted in 2.2.2 Navigation document above, the navigation features represent a universal and flexible navigation system.

The EPUB navigation document can also be reused in the body of an EPUB publication by including it in the spine. To avoid the situation in highly structured documents where it might not be desirable to display the complete table of contents to users in the body of the publication, the display level can be modified using the hidden attribute [epub-33]. This attribute is ignored by reading systems when they render the table of contents outside the spine (e.g., in their own specialized views), which avoids minimizing the information that is available.

EPUB creators are also encouraged to supply additional nav elements if their EPUB publications contain non-structural points of interest, such as figures, tables, etc., to further enhance access to the content.

[html] supports a number of elements that make markup more semantically meaningful (e.g., section, nav, and aside). EPUB creators are encouraged to use these elements, in conjunction with best practices for authoring well-structured web content, when creating EPUB XHTML content documents. These additions allow content to be better grouped and defined, both to represent the structure of documents and to facilitate their logical navigation. XHTML content documents also natively support the inclusion of ARIA role and state attributes and events, including the dedicated [dpub-aria-1.0] roles, enhancing the ability of Assistive Technologies to interact with the content.

EPUB 3 also includes the epub:type attribute [epub-33], which allows the inclusion of additional information to any element in an EPUB content document to express its purpose and meaning within the work. Refer to the section on Expressing Structural Semantics [epub-33] for more information.

At its core, EPUB is designed for dynamic layout: content is typically intended to be formatted on the fly rather than being typeset in a paginated manner in advance. This core capability is useful for optimizing rendering onto different-sized device screens or window sizes, and it facilitates and simplifies content accessibility.

While it is possible to incorporate more highly formatted content in EPUB — for example via bitmap images or SVG graphics, or even use of CSS explicit positioning and/or table elements to achieve particular visual layouts — EPUB creators are strongly discouraged from utilizing such techniques. These techniques are not reliable in EPUB since many reading systems render content in a paginated manner rather than creating a single scrolling viewport and since each reading system might define its own pagination algorithm. In general, it is preferable to achieve visual richness by using CSS Style Sheets without absolute sizing or positioning.

Aural renderings of content are important for accessibility and are a desirable feature for many users. A baseline to facilitate aural rendering is to utilize semantic HTML designed for dynamic layout. Refer to 2.10 Text-to-speech for more information on how to use the native facilities that XHTML content documents include.

Media overlays [epub-33] provide the ability to synchronize the text and audio content of an EPUB publication. Beyond benefiting accessibility, overlays have other applications, e.g., synchronizing text and audio as a tool for learning to read.

EPUB 3 adopts a progressive enhancement approach for scripted content, whereby scripting is not allowed to interfere with the integrity of the document (i.e., not result in information loss when scripting is not available). Consequently, although documents that do employ scripting can provide fallbacks [epub-33] to further facilitate access to their contents, the documents have to be accessible without them.

EPUB creators should always implement best practices for accessible scripting in web documents, such as provided in [wai-aria], and reserve the use of scripting for situations in which interactivity is critical to the user experience.