EPUB Reading Systems 3.3

Abstract

EPUB® 3 defines a distribution and interchange format for digital publications and documents. The EPUB format provides a means of representing, packaging, and encoding structured and semantically enhanced Web content — including HTML, CSS, SVG and other resources — for distribution in a single-file container.

This specification defines the conformance requirements for EPUB 3 Reading Systems — the user agents that render EPUB Publications.

This section is non-normative.

The EPUB 3 standard is separated into two distinct concerns: the authoring of EPUB Publications is defined in the core specification [EPUB-33], while this specification details the rendering requirements for them in EPUB Reading Systems.

An EPUB Reading System can take many forms. It might have a visual display area for rendering the content to users, for example, or it might only provide audio playback of the content. As a result, there is no single set of rules that all Reading Systems must follow. Rather, this specification breaks down the rendering requirements based on a Reading System's capabilities and the features its supports.

Moreover, this specification provides a great deal of flexibility to developers to create unique user interfaces, such as in their bookshelves. As a result, metadata processing requirements are often quite minimal, for example.

So, although this specification identifies the formal requirements for Reading Systems, it is not possible to read this document in isolation. Developers should also familiarize themselves with the full content structure of an EPUB Publication to understand the complete range of information that is available.

Note

A conforming Reading System is not necessarily a single dedicated program or device but might exist as a distributed system.

This specification uses terminology defined in EPUB 3.3 [EPUB-33] as well as the terms defined in this section. Terms appear capitalized wherever used.

Only the first instance of a term in a section links to its definition.

Content Display Area

The area within the Viewport dedicated to the display of EPUB Content Documents. The Content Display Area excludes any borders, margins, headers, footers, or other decoration a EPUB Reading System might inject into the Viewport.

In the case of synthetic spreads, the Viewport contains two Content Display Areas.

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words MAY, MUST, MUST NOT, OPTIONAL, RECOMMENDED, SHOULD, and SHOULD NOT in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.

This section is non-normative.

The [HTML] standard is continuously evolving — there are no longer versioned releases of it. That standard, in turn, references various technologies that continue to evolve, such as MathML, SVG, CSS, and JavaScript.

Reading System developers must keep track of the changes to HTML, and the technologies it references, to ensure they keep their systems up to date.

This specification does not require EPUB Reading Systems to support scripting, HTML forms or the HTML DOM. Reading Systems conformant with this specification are only expected to be able to process a conforming EPUB Content Document. As support for scripting and HTML forms is not compulsory, a conformant Reading System may not be a fully-conformant HTML user agent.

This specification does not reference a specific version of [SVG], but instead uses an undated reference. Whenever there is any ambiguity in this reference, the latest recommended specification is the authoritative reference.

This approach ensures that EPUB will always keep pace with changes to the SVG standard. Reading System developers must keep track of changes to the SVG standard to ensure they keep their systems up to date.

Reading Systems MUST process Publication Resources [EPUB-33].

Reading Systems MAY support an arbitrary set of Foreign Resource types, and MUST process fallbacks for unsupported Foreign Resources as defined in Foreign Resources [EPUB-33] if not.

Reading Systems SHOULD support remote resources, as defined in Resource Locations [EPUB-33].

Reading Systems MUST prevent data URLs [RFC2397] from opening in top-level browsing contexts [HTML], except when initiated through a Reading System affordance such as a context menu. If a Reading System does not use a top-level browsing context for Top-level Content Documents, for example if the Top-level Content Document is an SVG, it MUST also prevent data URLs from opening as though they are Top-level Content Documents.

If a Reading System has a Viewport, it MUST support the image Core Media Type Resources [EPUB-33].

If it has the capability to render prerecorded audio, it MUST support the audio Core Media Type Resources [EPUB-33].

Note

It is recommended that Reading Systems support at least one of the H.264 [H264] and VP8 [RFC6386] video codecs, but this is not a conformance requirement — a Reading System may support other video codecs, or none at all. Reading System developers should take into consideration factors such as breadth of adoption, video playback quality, and technology usage royalty requirements when making the choice to implement video in either format, or both.

A Reading System MUST be all of the following:

a conformant non-validating processor [XML].
a conformant processor as defined in [XML-NAMES].

In addition, when processing XML documents, it MUST NOT resolve external identifiers in DOCTYPE, ENTITY and NOTATION declarations [XML].

A Reading System MUST process, for all Publication Resources, the attributes that set the language for the documents' contents, when applicable. This includes:

the xml:lang attribute for all XML documents like the Package Document or the Media Overlay Document [EPUB-33].
the xml:lang and lang attributes for XHTML [HTML] or SVG [SVG].

Similarly, a Reading System MUST process, for all Publication Resources, the attributes that set the base direction for the documents' contents, when applicable. This includes:

the dir attribute for the Package Document [EPUB-33]. (See also 3.1 Base Direction for further details.)
the dir attribute for XHTML [HTML].
the direction attribute for SVG [SVG].

In the absence of this information in a Publication Resource, Reading Systems MUST NOT assume either the language or the base direction of that resource from information expressed in the Package Document (i.e., in xml:lang and dir attributes, in hreflang attributes on link elements, or from dc:language elements). Refer to a resource's formal specification for more information about to handle the absence of explicit language or direction information.

Reading Systems may support network access to enable the retrieval of remote resources and to allow Scripted Content Documents to communicate with web-hosted APIs and retrieve resources.

Providing network access, however, increases both the security risks to the Reading Systems and the security and privacy risks to users. These risks are often unique to Reading Systems and the platforms they run on — the browser cores most dedicated Reading System apps are built on do not offer the same security and privacy controls as web browsers themselves. Consequently, developers need to use extra caution when allowing network access, and more thoroughly test that their Reading Systems are not vulnerable to attacks. More information about these risks is provided in 15. Security and Privacy.

If Reading System developers opt to allow network access, it is strongly RECOMMENDED that they include methods to notify the user that network activity is occurring and/or that allow them to disable it.

For example, users might be presented with a global setting that disables all network access on a Reading System-wide or publication-by-publication basis (i.e., no downloading of remote resources or scripting access). Users could also be provided the option to block or terminate network access occurring within a publication to prevent unwanted scripting interactions.

Reading Systems SHOULD open links that resolve outside the EPUB Publication in a new browser instance to ensure that the security and privacy controls of the browser are available to users.

Although links to external web sites and resources are commonly found in EPUB Content Documents, these are not the only sources. If a Reading System provides access to linked records [EPUB-33] in the Package Document metadata, for example, it should similarly open the links in a new browser instance.

Note

For more information about the security and privacy issues related to external links, see 15.2 Threat Model.

Reading Systems MUST process the Package Document [EPUB-33].

If the dir attribute is set and indicates a base direction of ltr or rtl, Reading Systems MUST override the bidi algorithm per the higher-level protocols defined in [BIDI], setting the paragraph embedding level to 0 if the base direction is ltr, or 1 if the base direction is rtl.

Otherwise the base direction is auto, in which case Reading Systems MUST determine the text's direction by applying the Unicode Bidi Algorithm, beginning with Rule P2 of [BIDI].

Reading Systems SHOULD NOT depend on the Unique Identifier being unique to one and only one EPUB Publication. Determining whether two EPUB Publications with the same Unique Identifier represent different versions of the same publication, or different publications, may require inspecting other metadata, such as the last modification date, titles, and authors.

White Space

Reading Systems MUST strip and collapse ASCII whitespace [Infra] from Dublin Core [DC11] and meta element values [EPUB-33] before processing.

The identifier element

To determine whether an identifier conforms to an established system or has been granted by an issuing authority, Reading Systems SHOULD check for an identifier-type property [EPUB-33].

The title element

Reading Systems MUST recognize the first title element in document order as the main title of the EPUB Publication and present it to users before other title elements.

This specification does not define how to process additional title elements.

The language element

The language(s) of the EPUB Publication specified in language elements are informational. Some uses of this information include:

exposing it to the user through the Reading System user interface
using it to enhance functionality in a bookshelf (e.g., sorting by language)
using it to optimize the reading interface (e.g., to preload text-to-speech languages)

See 2.6 Internationalization for more details on how to determine the language of a Publication Resource.

The creator element

When determining display priority, Reading Systems MUST use the document order of creator elements in the metadata section, where the first creator element encountered is the primary creator. If a Reading System exposes creator metadata to the user, it SHOULD include all the creators listed in the metadata section whenever possible (e.g., when not constrained by display considerations).

The meta element

If a Reading System does not recognize the scheme attribute value, it SHOULD treat the value of the element as a string.

Reading Systems SHOULD ignore all meta elements whose property attributes define expressions they do not recognize. A Reading System MUST NOT fail when encountering unknown expressions.

If the property attribute's value does not include a prefix, Reading Systems MUST use the prefix URL "http://idpf.org/epub/vocab/package/meta/#" to expand the value.

The link element

Retrieval and support of linked resources is OPTIONAL.

The language identified in an hreflang is purely advisory. Upon fetching the resource, a Reading System must only use the language information associated with the resource to determine its language, not the metadata included in the link to the resource.

In the case of a linked metadata record [EPUB-33], Reading Systems MUST NOT skip processing the metadata expressed in the Package Document and only use the information expressed in the record. Reading Systems MAY compile metadata from multiple linked records.

When it comes to resolving discrepancies and conflicts between metadata expressed in the Package Document and in linked metadata records, Reading Systems MUST use the document order of link elements in the Package Document to establish precedence (i.e., metadata in the first linked record encountered has the highest precedence and metadata in the Package Document the lowest, regardless of whether the link elements occur before, within or after the package metadata elements).

Reading Systems MUST ignore any instructions contained in linked resources related to the layout and rendering of the EPUB Publication.

If any of the rel or properties attributes' values do not include a prefix, Reading Systems MUST use the prefix URL "http://idpf.org/epub/vocab/package/link/#" to expand the values.

Proprietary Metadata

It MUST ignore proprietary metadata properties that pertain to layout expressions if they conflict behaviorally with the property semantics defined in 6.1 Fixed-Layout Properties.

Reading Systems MAY optimize the rendering depending on the properties set in the properties attribute (e.g., disable a rendering process or use a fallback). Reading Systems MUST ignore values of the properties attribute they do not recognize.

Reading Systems SHOULD NOT use non-Publication Resources in the rendering of an EPUB Publication due to the inherent limitations and risks involved (e.g., lack of information about the resource and how to process it, security risks from remotely-hosted sources, lack of fallbacks, etc.).

A Reading System that does not support the Media Type of a given Publication Resource MUST traverse the fallback chain until it has identified at least one supported Publication Resource to use in place of the unsupported resource. If the Reading System supports multiple Publication Resources in the fallback chain, it MAY select the resource to use based on specific properties [EPUB-33] of that resource, otherwise it SHOULD honor the EPUB Creator's preferred fallback order. If a Reading System does not support any resource in the fallback chain, it MUST alert the reader that it could not display the content.

When manifest fallbacks [EPUB-33] are provided for Top-level Content Documents, Reading Systems MAY choose from the available options in order to find the optimal version to render in a given context (e.g., by inspecting the properties attribute for each).

Reading Systems MUST terminate the fallback chain at the first reference to a manifest item it has already encountered.

If any of the properties attribute's values do not include a prefix, Reading Systems MUST use the prefix URL "http://idpf.org/epub/vocab/package/item/#" to expand the values.

Reading Systems MUST provide a means of rendering the EPUB Publication in the order defined in the spine, which includes: 1) recognizing the first primary itemref as the beginning of the default reading order; and, 2) rendering successive primary items in the order given in the spine.

When a user traverses the default reading order defined in the spine [EPUB-33], a Reading System MAY automatically skip itemref elements marked as non-linear. When a user activates a hyperlink to a non-linear resource, however, Reading Systems MUST render the referenced resource or a designated fallback. Reading Systems MAY also provide the option for users to select whether to skip non-linear content by default or not.

If the EPUB Creator has not specified the page-progression-direction, the Reading System MUST assume the value of default. When the EPUB Creator has set the value of the page-progression-direction to default, either explicitly or implicitly, the Reading System can choose rendering direction.

If page-progression-direction has a value other than default, the Reading System MUST ignore any directionality computed from pre-paginated XHTML Content Documents.

If any of the properties attribute's values do not include a prefix, Reading Systems MUST use the prefix URL "http://idpf.org/epub/vocab/package/itemref/#" to expand the values.

Reading Systems MUST ignore all values expressed in spine itemref properties attributes that they do not recognize.

Reading Systems MUST NOT skip spine references to duplicate manifest items when rendering the linear reading order. The Reading System MUST treat these as distinct items for UI purposes (for example, each occurrence could be independently bookmarked or annotated). When a Reading System follows a hyperlink to a resource referenced multiple times in the spine, the Reading System MUST move to the position of the first occurrence of the document in the linear reading order.

When a spine itemref element's properties attribute contains an override of a global rendering property [EPUB-33], Reading Systems MUST follow the requirements for the override's global value when displaying that spine item.

For example, a spine item that contains the layout-pre-paginated override [EPUB-33] is rendered following the requirements of the global pre-paginated value.

If more than one override for the same property is specified in a properties attribute, Reading Systems MUST only process the first value listed in the attribute.

In the context of this specification, support for collections in Reading Systems is OPTIONAL. Reading Systems MUST ignore collection elements that define unrecognized roles.

The definition of EPUB Content Documents [EPUB-33] includes various authoring restrictions to optimize the cross-compatibility of content (e.g., prohibiting CSS for setting language and direction [EPUB-33]). Unless stated otherwise in this specification, Reading Systems MAY support these restricted features.

Reading Systems MUST process XHTML Content Documents [EPUB-33].

Unless explicitly defined in this section as overridden, Reading Systems MUST process XHTML Content Documents using semantics defined by the [HTML] specification and honor any applicable user agent conformance constraints expressed therein.

In addition to the requirements in 9. Processing Structural Semantics, a Reading System MUST ignore semantics expressed on the [HTML] head element or its descendants.

Reading System support for the attribute processing model [RDFA-CORE] is OPTIONAL.

Reading System support for the attribute processing model is OPTIONAL, as is the conversion to JSON [Microdata].

Use of the switch element is deprecated [EPUB-33]. Refer to its definition in [EPUBContentDocs-301] for implementation information.

Use of the trigger element is deprecated [EPUB-33]. Refer to its definition in [EPUBContentDocs-301] for implementation information.

Vendors MAY introduce functionality not defined in this specification to enhance the rendering of EPUB Publications.

To facilitate this experimentation, vendors MAY define custom attributes for use in XHTML Content Documents provided they are from a foreign namespace, which is defined as a namespace [XML-NAMES] that does not incorporate either of the following strings in its domain [URL]:

w3.org
idpf.org

Note

Note that this restriction also disallows the usage of custom attributes without a namespace prefix (e.g., <p foo="">…</p>). Indeed, such attributes are considered to be part of the default namespace, i.e., either the XHTML or SVG namespaces, which are both in the w3.org domain.

Custom attributes, and the behaviors associated with them, MUST NOT alter the integrity of an EPUB Publication.

Note

Custom attributes are usually defined in a Reading System-specific manner and are not intended for use by Reading Systems other than the one that they have been defined for. For generic extensions that are to be used by multiple independent Reading Systems, future versions of this specification should be extended to provide the feature explicitly.

To support MathML embedded in XHTML Content Documents, a Reading System:

MUST be an input-compliant processor for Presentation MathML, as defined in the [MATHML3] specification.
MUST, if it has a Viewport, support visual rendering of Presentation MathML.
MAY support rendering of Content MathML found in annotation-xml elements.

Note

Reading Systems may choose to use third-party libraries such as MathJax to provide MathML rendering.

Reading Systems MUST process SVG embedded in XHTML Content Documents as defined in 4.2 SVG Content Documents.

For the purposes of styling SVG embedded in XHTML Content Documents by reference, Reading Systems MUST NOT apply CSS style rules of the containing document to the referenced SVG document.

For the purposes of styling SVG embedded in XHTML Content Documents by inclusion, Reading Systems MUST apply applicable CSS rules of the containing document to the included SVG elements.

Note

SVG included by reference is processed as a separate document, and can include its own CSS style rules just like an SVG Content Document would. Note that this is consistent with situations where an [HTML] object element references an external [HTML] element.

Reading System support for the submission of [HTML] forms is OPTIONAL. A Reading System might, for example, prevent form submissions by limiting access to networking.

Reading Systems MUST process SVG Content Documents [EPUB-33].

To process SVG Content Documents and SVG embedded in XHTML Content Documents, a Reading System:

MUST process SVG Content Documents using semantics defined by the [SVG] specification, and honor any applicable user agent conformance constraints expressed therein, unless explicitly defined by this specification as overridden.
MUST meet the conformance criteria defined in 4.4 Scripting.
MUST, if it has a Viewport, support the visual rendering of SVG using CSS as defined in Styling [SVG] and it SHOULD support all properties defined in the Property Index [SVG]. In the case of embedded SVG, a Reading System MUST also conform to the constraints defined in 4.1.2.2.1 Embedded SVG and CSS.

If a Reading System has a Viewport, it MUST support the visual rendering of XHTML Content Documents via CSS [EPUB-33].

To support CSS, a Reading System:

MUST support the official definition of CSS as described in the [CSSSnapshot].
SHOULD support all applicable modules in [CSSSnapshot] that have reached at least Candidate Recommendation status [W3CProcess] (and are widely implemented).
MUST support [TrueType], [OpenType], [WOFF] and [WOFF2] font resources referenced from @font-face rules.
MUST support all prefixed properties defined in CSS Style Sheets — Prefixed Properties [EPUB-33].
SHOULD apply EPUB Creator style sheets as written to EPUB Content Documents.
SHOULD NOT override the EPUB Creator's style sheets, but SHOULD do so in a way that preserves the Cascade when necessary: through a user agent style sheet, the getOverrideStyle method [DOM-Level-2-Style], or [HTML] style attributes.
It MAY override parts of the EPUB Creator's style sheet because of user interaction.

In addition to supporting CSS properties as defined above, a Reading System's user agent style sheet SHOULD support the [HTML] suggested default rendering.

Reading System developers should implement CSS support at the level of major browsers and publicly document their user agent style sheets and how they interact with EPUB Creator's style sheets.

Reading Systems SHOULD support scripting [EPUB-33].

Reading System support for scripting depends on its usage context:

Reading Systems SHOULD support container-constrained scripting [EPUB-33] in reflowable EPUB Content Documents.
Reading Systems SHOULD support spine-level scripting [EPUB-33] in fixed-layout documents [EPUB-33].
Reading Systems SHOULD support spine-level scripting in reflowable EPUB Content Documents that use the "scrolled-doc" or "scrolled-continuous" [EPUB-33] presentation modes defined by the rendition:flow property. Similarly, if it supports spine-level scripting in reflowable EPUB Content Documents, it MUST implement the "scrolled-doc" presentation mode and SHOULD implement the "scrolled-continuous" presentation mode.
Reading Systems MAY support scripting in other contexts, but this specification does not address such scripting. As a result, the use of scripting in these contexts may not be consistent across Reading Systems.

If a Reading System supports scripting:

It MAY render Scripted Content Documents as an interactive, scripted user agent according to [HTML].
It MUST assign a unique origin [URL], shared by all spine-level scripts of the EPUB Publication. That origin [URL] MUST be unique for each user-specific instance of an EPUB Publication in a Reading System.
It MUST NOT allow a container-constrained script to modify the DOM of the parent Content Document or other contents in the EPUB Publication and MUST NOT allow it to manipulate the size of its containing rectangle. (Note: Even if a script is not container-constrained, the Reading System MAY impose restrictions on modifications (see also the dom-manipulation feature).)
It MAY place additional limitations on the capabilities provided to scripts during execution (e.g., limiting networking).
It MUST extend the navigator object [HTML] with the epubReadingSystem interface defined in 16. epubReadingSystem Object. It also MUST support the dom-manipulation and layout-change features defined in 16.4.1.2 Features in container-constrained scripting contexts.

Editor's note

Note that the definition of epubReadingSystem is currently marked as "at risk". If, in the final version of this document, the object becomes non-normative, then each "MUST" statement in the last bullet item would become a "MAY".

If a Reading System does not support scripting, it MUST process fallbacks for scripted content as defined in Fallbacks for Scripted Content Documents [EPUB-33].

Scripts may save persistent data through cookies and DOM storage, but Reading Systems MAY block such attempts. Reading Systems that allow users to store data MUST ensure they do not make that data available to other unrelated documents (e.g., ones that could be spoofed). In particular, checking for a matching document identifier (or similar metadata) is not a valid method to control access to persistent data.

Reading Systems that allow local storage SHOULD provide methods for users to inspect or delete that data.

Reading Systems SHOULD follow the DOM Event model as per [HTML] and pass UI events to the scripting environment before performing any default action associated with these events.

Reading System developers must ensure that scripts cannot disable critical functionality (such as navigation) to constrain the extent to which a potentially malicious script could impact their Reading Systems. As a result, although the scripting environment should be able to cancel the default action of any event, some events either might not be passed through or might not be cancelable.

This section is non-normative.

Reading System developers who also support scripting must be aware of the security issues that arise when Reading Systems execute scripted content. As the underlying scripting model employed by Reading Systems and browsers is the same, developers must take into consideration the same kinds of issues encountered in Web contexts.

Each Reading System must establish if it can trust the scripts in a particular document. Reading Systems should treat all scripts as untrusted (and potentially malicious), and developers should examine all vectors of attack and protect against them. In particular, developers should consider the following:

an attack against the runtime environment (e.g., stealing files from a user's hard drive);
an attack against the Reading System itself (e.g., stealing a list of a user's books or causing unexpected behavior);
an attack of one Content Document against another (e.g., stealing data that originated in a different document);
an attack of an unencrypted script against an encrypted portion of a document (e.g., an injected malicious script extracting protected content);
an attack against the local network (e.g., stealing data from a server behind a firewall).

To limit the possible damage of untrusted scripts, this specification recommends that Reading Systems establish a unique origin [URL] allocated to each EPUB Publication (see 7.1.1 URL of the Root Directory). Assigning a unique origin ensures that spine-level scripts [EPUB-33] are isolated from other EPUB Publications, and limits access to cookies, DOM storage, etc.

Examples of Web APIs that are tied to the concept of "origin" include Web Storage [WEBSTORAGE] and IndexedDB [INDEXEDDB], which EPUB Content Documents can interact with via scripting. Reading Systems that allow users to add/remove publications from a managed library (their "bookshelf") may maintain the publication's unique origin when the publication is removed and subsequently re-imported into the content library. Conversely, Reading Systems may create a new unique origin for every newly added publication.

This specification also recommends that container-constrained scripts [EPUB-33] not be allowed to modify the DOM of the parent EPUB Content Document and/or manipulate the containing rectangle (see 4.4 Scripting).

Note that compliance with these recommendations does not guarantee protection from the possible attacks listed above; developers must examine each potential vulnerability within the context of their Reading System.

Reading Systems MUST support the rendering of Fixed-Layout Documents [EPUB-33].

The default value reflowable MUST be assumed by EPUB Reading Systems as the global value if no meta element carrying this property occurs in the metadata section section [EPUB-33].

When the rendition:layout property is set to pre-paginated, Reading Systems MUST NOT include space between the adjacent content slots when rendering Synthetic Spreads.

The rendition:layout property values have the following processing requirements:

reflowable: Reading Systems MAY apply dynamic pagination when rendering.
pre-paginated: Reading Systems MUST produce exactly one page per spine itemref [EPUB-33] when rendering.

When a spine itemref element's properties attribute contains an override of the global rendition:layout property [EPUB-33], Reading Systems MUST follow the requirements for the override's global value when displaying that spine item (e.g., a spine item that specifies layout-prepaginated is rendered following the requirements of the global pre-paginated value).

The default value auto MUST be assumed by EPUB Reading Systems as the global value if no meta element carrying this property occurs in the metadata section.

The rendition:orientation property values have the following processing requirements:

auto: The Reading System determines the orientation in which to render the content.
landscape: Reading Systems that support multiple orientations SHOULD render the content in lanscape orientation.
portrait: Reading Systems that support multiple orientations SHOULD render the content in portrait orientation.

The means by which they convey the intent is implementation specific.

The default value auto MUST be assumed by EPUB Reading Systems as the global value if no meta element carrying this property occurs in the metadata section.

The rendition:spread property values have the following processing requirements:

none: Reading Systems MUST NOT incorporate spine items in a Synthetic Spread. Reading Systems SHOULD create a single viewport positioned at the center of the screen.
landscape: Reading Systems SHOULD render a Synthetic Spread for spine items only when the device is in landscape orientation.
portrait (deprecated): Reading Systems SHOULD treat the value "portrait" as a synonym of "both" and create spreads regardless of orientation.
both: Reading Systems SHOULD render a Synthetic Spread regardless of device orientation.
auto: Reading Systems MAY use Synthetic Spreads in specific or all device orientations as part of a Content Display Area utilization optimization process.

The rendition:page-spread-left property indicates that the given spine item SHOULD be rendered in the left-hand slot in the spread, and rendition:page-spread-right that it SHOULD be rendered in the right-hand slot.

Reading Systems that support the spread-none property MUST recognize the rendition:page-spread-center as an alias for it, otherwise they MUST ignore the rendition:page-spread-center property.

The rendition:page-spread-left and rendition:page-spread-right properties apply to both pre-paginated and reflowable content, and they only apply when the Reading System is creating Synthetic Spreads.

The rendition:page-spread-* properties MUST take precedence over whatever value of the page-break-before property [CSSSnapshot] has been set for an XHTML Content Document.

When a reflowable spine item follows a pre-paginated one, the reflowable one SHOULD start on the next page — as defined by the page-progression-direction [EPUB-33] — when it lacks a rendition:page-spread-* property value.

If the reflowable spine item has a rendition:page-spread-* specification, it MUST be honored (e.g., by inserting a blank page).

Similarly, when a pre-paginated spine item follows a reflowable one, the pre-paginated one SHOULD start on the next page (as defined by the page-progression-direction) when it lacks a rendition:page-spread-* property value. If the pre-paginated spine item has a rendition:page-spread-* specification, it MUST be honored (e.g., by inserting a blank page).

When a Reading System encounters two spine items that represent a true spread (i.e., two adjacent spine items with the rendition:page-spread-left and rendition:page-spread-right properties), it SHOULD create the spread with no space between the adjacent pages.

XHTML

Reading Systems MUST use the width and height expressions as defined in Expressing in HTML [EPUB-33] to render XHTML Content Documents.

Reading Systems MUST clip XHTML content to the initial containing block (ICB) dimensions declared in the viewport meta tag — content positioned outside of the initial containing block will not be visible. When the ICB aspect ratio does not match the aspect ratio of the Reading System Content Display Area, Reading Systems MAY position the ICB inside the area to accommodate the user interface; in other words, added letter-boxing space MAY appear on either side (or both) of the content.

SVG

Reading Systems MUST use the dimensions as defined in Expressing the ICB in SVG [EPUB-33] to render SVG Content Documents.

When rendering Fixed-Layout Documents, the default intent is that the Content Display Area SHOULD occupy as much of the available Viewport area as possible. Reading Systems SHOULD NOT inject additional content such as border, margins, headers, or footers into the Viewport.

Note

This specification does not define how the initial containing block [CSS2] is placed within the Reading System Content Display Area.

Note

The exposure of Reading System control widgets to the user is implementation-specific and not included in the above behavioral expectations.

Reading Systems MUST process the EPUB Container [EPUB-33].

Note

An application that processes OCF Containers does not have to be a full-fledged Reading System (e.g., an application might only extract the content of a container or check the validity of the packaged content). In these cases, developers of such applications can ignore the rendering requirements for Reading Systems defined in this section.

Reading Systems MUST assign a URL [URL] to the Root Directory of the OCF Abstract Container. This URL is called the container root URL. It is implementation specific, but the implementation MUST have the following properties:

The result of parsing [URL] "/" with the container root URL as base is the container root URL.
The result of parsing [URL] ".." with the container root URL as base is the container root URL.
The origin of the container root URL is unique for each user-specific instance of an EPUB Publication in a Reading System.

Note

The unicity of the origin per each user-specific instance of an EPUB Publication in a Reading System means that if two different users acquire a copy of the same EPUB Publication, the origins will be different for the two users on those copies even if the same Reading System is used.

Note

The required properties of the container root URL are such that it behaves similarly to a URL defined as follows:

URL component	Values
scheme	`http` or `https`
host	`localhost`
port	a dynamic port uniquely assigned to the EPUB instance

for example:

Container File	File Path	URL
Root Directory	`empty string`	`http://localhost:49152/`
Package Document	`OPS/package.opf`	`http://localhost:49152/OPS/package.opf`
Content Document	`HTML/file name.xhtml`	`http://localhost:49152/HTML/file%20name.xhtml`

Note

Some language specifications reference Requests For Comments that preceded [URL], in which case the earlier RFC applies for content in that particular language.

Note

Unlike most language specifications, Reading Systems must use the container root URL as the base URL [URL] for all files within the META-INF directory. See also the section on Parsing URLs in the META-INF Directory in [EPUB-33].

Although EPUB Creators are required to follow various File Name and File Path restrictions [EPUB-33] for maximum interoperability, Reading Systems SHOULD attempt to process File Names and Paths that are not valid to these requirements. Invalid File Names and Paths may only be problematic on some operating systems.

This specification does not specify how a Reading System that is unable to represent OCF File Names and Paths would compensate for this incompatibility.

If a Reading System cannot preserve the names of files during an unzipping process, it will have to compensate for any name translation that took place in the content (i.e., in any URIs that reference the original name).

Container File (container.xml): A Reading System MUST, by default, use the Package Document referenced from first rootfile element to render the EPUB Publication. If the Reading System recognizes a means of selecting from the other available options, it MAY choose a more appropriate Package Document.
Metadata File (metadata.xml): Reading Systems SHOULD ignore metadata.xml files with unrecognized root elements.
Encryption File (encryption.xml): Reading Systems MUST only decrypt data encrypted after signing before computing the digest used to validate the signature.
Manifest File (manifest.xml): Reading Systems MUST NOT use ancillary manifest information contained in the ZIP archive or in the manifest.xml file for processing an EPUB Publication.
Other Files: Reading Systems MUST NOT fail when encountering configuration files in the META-INF directory not listed in Reserved Files [EPUB-33].

A Reading System that supports the OCF Container:

MUST treat any OCF Containers that specify the ZIP file is split across multiple storage media as in error.
MUST treat any OCF Containers that use compression techniques other than Deflate as in error.
MUST support the ZIP64 extensions defined as "Version 1".
MUST treat OCF ZIP Containers that use ZIP encryption features as in error.
MAY generate a physical directory for the Root Directory of the OCF Abstract Container if it unzips the contents.
does not have to preserve information from an OCF ZIP Container through load and save operations outside the context of the OCF Abstract Container. In particular, a Reading System does not have to preserve CRC values, comment fields or fields that hold file system information corresponding to a particular operating system (e.g., External file attributes and Extra field).

With respect to specific fields in the OCF ZIP Container archive, the Reading System:

MUST treat version needed to extract field values other than 10, 20 or 45 in the local file header table as being in error.
MUST treat compression method field values other than 0 or 8 in the local file header table as being in error.
MUST treat OCF ZIP Containers with an Archive decryption header or an Archive extra data record as being in error.

Reading Systems SHOULD support deobfuscation of fonts as defined in Font Obfuscation [EPUB-33].

For Reading Systems to get the original obfuscated data back, simply reverse the process: the source file becomes the obfuscated data, and the destination file contains the raw data.

Note

EPUB 3 allowed the obfuscation of fonts prior to EPUB 3.0.1, but did not specify the order of obfuscation and compression. As a result, Reading Systems might encounter invalid fonts after decompression and deobfuscation. In such instances, deobfuscating the data before inflating it may return a valid font. Reading Systems do not have to support this method of retrieval, but developers must consider it when supporting EPUB 3 content generally.

Reading Systems with the capability to render prerecorded audio SHOULD support Media Overlays [EPUB-33].

If a Reading System does not support Media Overlays, it MUST ignore both the media-overlay attribute on Manifest item elements [EPUB-33] and the manifest item elements where the media-type attribute value equals application/smil+xml.

When a Reading System loads a Package Document, it MUST refer to the Manifest item elements' [EPUB-33] media-overlay attributes to discover the corresponding Media Overlays for EPUB Content Documents.

Reading Systems MUST support playback for XHTML Content Documents, and MAY support SVG Content Documents.

Playback MUST start at the Media Overlay element which corresponds to the desired EPUB Content Document starting point. Note that the start of an EPUB Content Document MAY correspond to an element at the start or in the middle of a Media Overlay. When the Media Overlay Document finishes playing, the Reading System SHOULD load the next EPUB Content Document (as specified in the Package Document spine) and also load its corresponding Media Overlay Document, provided that one is given.

Reading Systems MUST render immediate children of the body element [EPUB-33] in a sequence. A seq element's [EPUB-33] children MUST be rendered in sequence, and playback completes when the last child finishes playing. Reading System MUST render a par element's [EPUB-33] children in parallel (with each starting at the same time), and playback completes when all the children finish playing. Reading System playback of the Media Overlay Document completes when the body element's last child finishes playing.

When presented with a Media Overlay audio element [EPUB-33], Reading Systems MUST play the audio resource referenced by the src attribute, starting at the clip offset time given by the clipBegin attribute [EPUB-33] and ending at the clip offset time given by the clipEnd attribute [EPUB-33].

In addition:

If the EPUB Creator has not specified a clipBegin, Reading Systems MUST assume the value "0".
If the EPUB Creator has not specified a clipEnd, Reading Systems MUST assume the value to be the full duration of the physical media.
If clipEnd exceeds the full duration of the physical media, Reading Systems MUST assume its value to be the full duration of the physical media.

User-controllable audio playback options SHOULD include timescale modification, in which the user can alter the playback rate without distorting the pitch. The suggested range is half-speed to double-speed.

When presented with a Media Overlay text element [EPUB-33] whose src attribute references a target element [HTML] or SVG Fragment Identifier [SVG], Reading Systems SHOULD ensure the referenced element is visible in the Viewport. Reading Systems MAY support other fragment identifier schemes.

During Media Overlays playback, Reading Systems with a Viewport SHOULD add the class names given by the metadata properties active-class and playback-active-class [EPUB-33] to the appropriate elements, when specified, in the EPUB Content Document. Conversely, the class names SHOULD be removed when the playback state changes, as described in Associating Style Information [EPUB-33].

The active-class and playback-active-class metadata properties are OPTIONAL, and if omitted, Reading System behavior is implementation-specific.

Reading System behavior when a fragment identifier [EPUB-33] does not reference an element is also implementation-specific.

Because the Media Overlay is closely linked to the EPUB Content Document, it is very easy for Reading Systems to locate a position in the EPUB Content Document based on the current position in the Media Overlay playback. If the user pauses synchronized playback and navigates to a different part of the EPUB Publication, synchronized playback MUST resume at that point. For example, if a specific page number in the EPUB Content Document is the desired location, then this same point is located in the Media Overlay and playback started there.

This same approach allows for synchronizing the Media Overlay playback with user selection of a navigation point in the EPUB Navigation Document. The Reading System loads the Media Overlay for that file and finds the correct point for starting playback based on the ID [XML] of the navigation point target.

Note

EPUB Creators may associate a Media Overlay Document directly with a Navigation Document in order to provide synchronized playback of its contents, regardless of whether the XHTML Content Document in which it resides is included in the spine. See EPUB Navigation Document [EPUB-33] for more information.

Note

EPUB Creators may associate Media Overlay Document elements with EPUB Content Document structures such as tables. Reading Systems should ensure that Media Overlay playback remains synchronized with user navigation of table rows and cells. The Reading System might also play the corresponding table header preceding the contents of the cell.

An EPUB Content Document with which a Media Overlay is associated MAY itself contain embedded video and audio media. The Media Overlay elements MAY point to these elements. Unlike text and images, video and audio media have an intrinsic duration. Consequently, when a Reading System renders the synchronization described by a Media Overlay, it MUST override the default playback behavior of audio and video media embedded within the associated EPUB Content Document.

Note that the rules below apply only to referenced [HTML] video or audio elements within the associated EPUB Content Document. That is to say, the rules apply to only those elements pointed to by text elements [EPUB-33] within the Media Overlay (i.e., via the src attribute). These rules do not apply to embedded media not referenced by Media Overlay elements.

Reading Systems MUST deactivate the public playback interface for all referenced audio and video media embedded within an EPUB Content Document (typically: play/pause control, time slider, volume level, etc.). This behavior avoids interference between the scheduled playback sequence defined by the Media Overlay, and the arbitrary playback behavior due to user interaction or script execution. As a result, when the Reading System is in playback mode, it SHOULD:
- Hide the individual video/audio UI controls from the page, which overrides the default behavior defined by the [HTML] controls attribute.
- Prevent scripts embedded within the EPUB Content Document from invoking the JavaScript audio/video playback API (i.e., authored as part of the default behavior).
Reading Systems MUST initialize all referenced audio and video media embedded within an EPUB Content Document to their "stopped" state, and ready them to play from the zero-position within their content stream (possibly displaying the image specified using the [HTML] poster attribute). This requirement overrides the default behavior defined by the [HTML] autoplay attribute.
When an EPUB Content Document element becomes active, the CSS Style Sheet visual highlighting rules apply regardless of the content type referred to by that element's src attribute (e.g., the CSS class name defined by the active-class metadata property [EPUB-33] SHOULD be applied to visible video and audio player controls within the host EPUB Content Document).
In addition to the default behavior of Media Overlay activation for textual fragments and images, audio and video playback MUST be started and stopped according to the duration implied by the authored Media Overlay synchronization (as per the standard [SMIL3] timing model). There are two possible scenarios:
- When a Media Overlay text element has no audio [EPUB-33] sibling within its par [EPUB-33] parent container, the referenced EPUB Content Document audio or video media MUST play until it ends, at which point the text element's lifespan terminates. In this case, the implicit duration of the text element (and by inference, of the parent par container) is that of the referenced audio or video clip.
- When a Media Overlay text element has an audio sibling within its par parent container, Reading Systems MUST constrain the playback duration of the referenced EPUB Content Document audio or video media by the duration of the audio sibling. In this case, the actual duration of the parent par container is that of the child audio clip, regardless of the duration of the video or audio media pointed to by the text element. This behavior can result in embedded video or audio media ending playback prematurely (before reaching its full duration), or ending before the playback of the parallel Media Overlay audio is finished (in which case the last-played video frame SHOULD remain visible until the parent par container finally ends). This behavior is equivalent of the Media Overlay audio element implicitly carrying the behavior of the [SMIL3] endsync attribute.
  
  Furthermore, Reading Systems SHOULD expose user controls for the volume levels of each independent audio track (i.e., from the audio element of the Media Overlay, and from the embedded audio or video media within the EPUB Content Document), so that users can adjust the audio output to match their requirements. Note that having overlapping audio tracks is typically an authoring-time concern: content producers usually add a layer of audio information over a video track for description purposes. Reading Systems handling of simultaneous volume levels in any specific way is OPTIONAL.
When a text element becomes inactive in the Media Overlay, and when it points to embedded video or audio media, that referenced media MUST be reset to its initial "stopped" state, ready to be played from the zero-position within their content stream (possibly displaying the poster image specified using the [HTML] markup).

When a Media Overlay text element [EPUB-33] with no audio [EPUB-33] sibling element references text within the target EPUB Content Document, Reading Systems capable of text-to-speech (TTS) playback SHOULD render the referenced text using TTS.

Reading Systems SHOULD use the speech-related information provided in the target EPUB Content Document to play the audio stream as part of the Media Overlay rendering.

Note

See EPUB 3 Text-to-Speech Support [EPUB-TTS-10] for more information about supporting TTS technologies in EPUB Publications.

The Media Overlay text element's lifespan corresponds to the rendering time of the associated speech synthesis. The implicit duration of the text element (and by inference, of the parent par element) is therefore determined by the execution of the Text-to-Speech engine, and cannot be known at authoring time (factors like speech rate, pauses and other prosody parameters influence the audio output).

Reading Systems SHOULD use the semantic information provided by Media Overlay elements' epub:type attribute to determine when to offer users the option of skippable features.

Reading Systems SHOULD allow escaping of nested structures. Reading Systems MUST determine the start of nested structures by the value of the epub:type attribute and SHOULD offer users the option to skip playback of that structure and resume with whatever content comes after it.

Reading Systems SHOULD process the general package rendering properties [EPUB-33].

If a Reading System supports the specified rendering, it SHOULD use that method to handle overflow content, but MAY provide the option for users to override the requested rendering.

The default global value is auto if no meta element carrying this property occurs in the metadata section [EPUB-33]. Reading Systems MAY support only this default value.

The rendition:flow property values have the following processing requirements:

paginated: The Reading System SHOULD dynamically paginate all overflow content.
scrolled-continuous: The Reading System SHOULD render all Content Documents such that overflow content is scrollable, and SHOULD present the EPUB Publication as one continuous scroll from spine item to spine item (except where locally overridden [EPUB-33]).
scrolled-doc: The Reading System SHOULD render all Content Documents such that users can scroll overflow content, and SHOULD present each spine item as a separate scrollable document.
auto: The Reading System MAY render overflow content using its default method or a user preference, whichever is applicable.

For the rendition:flow-scrolled-continuous property, the scroll direction MUST be defined relative to the block flow direction of the root element of the XHTML Content Document referenced by the itemref element [EPUB-33]. The scroll direction MUST be vertical if the block flow direction is downward (top-to-bottom). It MUST be horizontal if the block flow direction of the root element is rightward (left-to-right) or leftward (right-to-left).

Reading Systems MUST ignore the rendition:flow property and its overrides when processing pre-paginated spine items [EPUB-33].

When the rendition:align-x-center property is set on a spine item, Reading Systems SHOULD render the content centered horizontally within the Viewport or spread, as applicable. This property does not affect the rendering of the spine item, only the placement of the resulting content box.

For reflowable content, Reading Systems that support this property MUST center each virtual page.

This specification does not define a default rendering behavior when Reading Systems do not support this property or EPUB Creators do not specify it. Reading Systems MAY render spine items by their own design.

This section is non-normative.

The primary focus of this specification is on how to process and render EPUB Publications and Reading Systems generally have little influence over the accessibility of the underlying content that they render (i.e., the accessibility is determined both by the capabilities of the technologies used and how they are taken advantage of by EPUB Creators). So long as Reading Systems fully and accurately support EPUB's required technologies, there should not be issues in the content rendering itself.

Where accessibility intersects with Reading Systems is in the user interface provided to users. But even here the specification provides a lot of leeway in terms of how to construct such interfaces as Reading Systems may take many forms. EPUB Reading Systems run the gamut from standalone applications to device-integrated programs (e.g., eInk readers) to web-hosted applications. A reading system may or may not even have a visual interface as auditory applications allow users who are blind to easily consume EPUB Publications.

Due to this flexibility, this specification does not mandate specific user interfaces that all Reading Systems must offer. At the same time, however, it is critically important that Reading Systems be as accessible as possible to ensure the widest number of users can read their content.

The following list outlines some of the common areas where a lack of accessibility impacts the reading experience for users.

Bookshelf — Ensure that the process for accessing and opening the user's content is accessible. For example, do not rely on a visual-only display like cover images. Exposing text representations of the title(s) and author(s), including the language they are expressed in, allows assistive technologies to properly announce the options.
User Interface Controls — Ensure that all controls (e.g., search boxes, annotation controls, and buttons to load features such as the table of contents) are exposed to assistive technologies and their actions do not rely on specific modalities (e.g., they only operate through touch or a mouse).
Search — Search access to the full text content of all EPUB Content Documents (including alternative text attributes, text in SVG images, etc.) is necessary to ensure users can locate information easily.
Display Control — Provide methods for users to tailor the styling of the content to their preferences (e.g., to change and increase fonts, increase line and word spacing, and apply alternate contrasts).
Zoom — Allow users to zoom the content to better size it to their needs, especially for Fixed-Layout Documents.
Reading Control — Ensure that the ability to read from page-to-page and document-to-document does not depend on physical interaction (i.e., users of assistive technologies can read the content without getting trapped at the end of pages or documents).
API Integration — Ensure that the accessibility tree, including support for roles, states and properties [WAI-ARIA-11], is exposed to the underlying operating system's accessibility API so that users can fully interact with the content when using assistive technologies.
Document Object Model (DOM) — Provide access to the [DOM] of EPUB Content Documents so that users can investigate the semantics and structure used in the source.
Table of Contents — Ensure that users can access the link text, including text alternatives for visual content. Activating the table of contents links should be possible using different inputs.

The DAISY Consortium maintains an accessibility test suite to aid in evaluating these issues and more.

In addition, although focused on browser acccessibility, the W3C's User Agent Accessibility Guidelines [UAAG20] provides many useful practices developers can apply to improve their Reading Systems.

This section is non-normative.

The particularity of an EPUB Publication is its structure. The EPUB format provides a means of representing, packaging, and encoding structured and semantically enhanced web content — including HTML, CSS, SVG, and other resources — for distribution in a single-file container.

For Reading Systems, this means that the security and privacy issues are primarily based on the features of those formats, and closely mirror the threats presented by web content generally.

Reading System developers also have a dual responsibility of both ensuring the security and privacy of their applications and helping limit the threats to users from the content that renders within them. The rest of this section explores the risk model of EPUB 3 with the aim of helping developers recognize and mitigate these risks.

Note

For the risks associated with the authoring of EPUB Publications, refer to the security and privacy section of [EPUB-33].

The greatest threats to users come from the content they read [EPUB-33], and the first line of defence against these attacks is the Reading Systems they use. Users expect that Reading Systems act as safeguards against malicious content and are often unaware that EPUB Publications are susceptible to the same security risks as web sites.

But although Reading Systems are relied on to provide security and privacy, they can also pose unintended threats to users depending on how information is handled. Tracking user information to optimize experiences is a common need, for example, but done without user permission and Reading Systems can run afoul of legal privacy requirements.

This section outlines some of the key threats that Reading System developers must take into consideration, with further details and recommendations in the following sections.

Scripting

Malicious scripts present several attack vectors against reading systems. If local storage is not secure, for example, they can attempt to compromise the user's data. Provided with network access, they may attempt unauthorized collection of user data.

More detailed discussion of these issues and how to mitigate them is provided in 4.4.3 Security Considerations.

Malicious content

EPUB Publications may contain resources designed to exploit security flaws in Reading Systems or the operating systems they run on.

Remote resources

Remote resources present the same risks as any EPUB Publication loaded from an untrusted source. Even if the publisher of the EPUB Publication is trusted, remote resources may be compromised.

Calls to remote resources can also be used to track information about users (e.g., through server logs). Reading Systems should limit the information they expose through HTTP requests to only what is essential to obtain the resource.

The origin of an EPUB is both unknown to the EPUB Creator and specific to each Reading System implementation. Consequently, if the EPUB Creator hosts remote resources on a web server they control, the server effectively cannot use security features that require specifying allowable origins, such as headers for CORS, Content-Security-Policy, or X-Frame-Options.

External links

Like remote resources, external links may be used to trick unwitting users into opening malicious resources on the web designed to exploit the Reading System or operating system.

Likewise, Reading Systems should also avoid exposing potentially identifying information about users through the traversal of links (e.g., not using tracking identifiers or exposing unnecessary information about the user's environment).

User-generated content

User-generated content within a reading experience, for example through text areas or other interactive components, could pose a threat to reading system security or user privacy if the content is not properly secured. More details on scripting security are provided in 4.4.3 Security Considerations.

Collection of user data

Collecting information about the user and their reading habits without obtaining their permission can violate privacy laws, even if the information is only intended for internal use.

Attempting to profile users based on their reading habits or through metadata in their publication (e.g., accessibility preferences) can expose users to unintended harms.

Additionally, the security considerations that apply to XML [XML] and ZIP [ZIP] files also apply to the Package Document and the Open Container Format, respectively. See the "Security Consideration" sections of the Media Type registrations for the application/oebps-package+xml and the application/epub+zip formats for further details.

Arguably the strongest measure that Reading System developers can take for privacy is to specify the data they intend to collect and use about the user and/or their reading behaviour and seek the consent of users to obtain it. They should also allow personalization and control over this information.

If a Reading System allows users to store persistent data, especially personally identifiable information, it must treat that data as sensitive.

It is understood that the collection of some user data may be required for the sale, delivery, and operation of an EPUB Publication, particularly on platforms where the sale of an EPUB Publication and the method of reading it are connected. In these cases, it is recommended that the Reading System or retailer be clear about the data being collected, how it is used, and allow for user opt-outs where possible. Anonymization of data is strongly recommended for the privacy and the security of the user and Reading System.

It is also understood that user data may be required or helpful for some Reading System affordances. In these cases, anonymization is strongly recommended. It is also recommended that Reading Systems inform users of what data is needed, what it is to be used for, and to provide methods to opt-out.

Content processors — defined as entities that handle the ingestion of EPUB content for distribution, display, or sale — should also be aware of the potential risks in ingestion. It is advised that content processors check content for malicious content on ingestion, in addition to the validation steps that usually occur. This could include running virus scans, validating external links and remote resources, and other precautions.

(Feature at Risk) Issue 1: Are there enough implementations?

Although this section is normative, it is not clear whether implementations of this interface are widespread enough to warrant this status. If it turns out not to be the case, this section will be marked as non-normative in the final version of the specification.

Note

Reading Systems act as the core rendering engines of EPUB Publications and provide a scripting environment based on the [DOM] specification. So, although this interface definition uses the [WEBIDL] notation for implementation by Reading Systems, Web browsers generally do not have to implement these objects.

This specification extends the the Navigator object [HTML] as follows.

WebIDL[Exposed=(Window)]
interface EpubReadingSystem {
    [LegacyUnforgeable] readonly attribute DOMString name;
    [LegacyUnforgeable] readonly attribute DOMString version;
    boolean hasFeature(DOMString feature, optional DOMString version);
};

partial interface Navigator {
    [LegacyUnforgeable, SameObject] readonly attribute EpubReadingSystem epubReadingSystem;
};

Note

This specification does not define an epubReadingSystem property extension for the WorkerNavigator object [HTML]. Reading Systems therefore do not have to expose the epubReadingSystem object in the scripting context of Workers, and EPUB Creators cannot rely on its presence.

The Navigator.epubReadingSystem object provides an interface through which a Scripted Content Document can query information about a user's Reading System.

The object exposes properties of the Reading System (its name and version), and provides the hasFeature method which can be invoked to determine the features it supports.

Example 1: JavaScript function to display the name of the current Reading System

alert("Reading System name: " + navigator.epubReadingSystem.name);

Reading Systems MUST expose the epubReadingSystem object on the navigator object of all loaded Scripted Content Documents, including any nested container-constrained scripting contexts [EPUB-33]. Reading Systems MUST ensure that the epubReadingSystem object is available no later than when the DOMContentLoaded event is triggered [HTML].

Note

Reading systems implementations may create cloned instances of the epubReadingSystem object in Scripted Content Documents for technical feasibility reasons. In such cases, the Reading System must ensure they consistently maintain the object's state — as reflected by the values of its properties and methods — across all copied instances.

The Reading System MUST make the following properties available for retrieving information about it.

Name	Description
`name`	Returns a `String` value representing the name of the Reading System (e.g., "`iBooks`", "`Kindle`").
`version`	Returns a `String` value representing the version of the Reading System (e.g., "`1.0`", "`2.1.1`").

This specification used to define a layoutStyle property, but it is now deprecated [EPUB-33]. Refer to its definition in [EPUBContentDocs-301] for more information.

The hasFeature method returns a boolean value indicating whether the Reading System supports any version of the specified feature, or undefined if the Reading System does not recognize the specified feature.

The optional version parameter allows EPUB Creators to query custom features that could change in incompatible ways over time. The return value indicates support only for the specified version of the feature.

Features defined in this specification are versionless. If a Reading System supports a feature defined in this specification, it MUST ignore any supplied version parameter and return a true value.

Example 2: JavaScript function to display support for scripted manipulation of the DOM

var feature = "dom-manipulation";

var conformTest = navigator.epubReadingSystem.hasFeature(feature);

alert("Feature " + feature + " supported?: " + conformTest);

The following table lists the set of features that Reading Systems that support the epubReadingSystem object MUST recognize. When the features are queried from the hasFeature method, Reading Systems MUST return a boolean value indicating their support.

Name	Description
`dom-manipulation`	Scripts may make structural changes to the document’s DOM (applies to spine-level scripting [EPUB-33] only).
`layout-changes`	Scripts may modify attributes and CSS styles that affect content layout (applies to spine-level scripting [EPUB-33] only).
`touch-events`	The device supports touch events, and the Reading System passes touch events to the content.
`mouse-events`	The device supports mouse events, and the Reading System passes mouse events to the content.
`keyboard-events`	The device supports keyboard events, and the Reading System passes keyboard events to the content.
`spine-scripting`	Indicates whether the Reading System supports spine-level scripting [EPUB-33] (e.g., so a container-constrained script [EPUB-33] can determine whether any actions that depend on scripting support in a Top-level Content Document have any chance of success before attempting them).

Reading System developers MAY add additional features, but future versions of this specification could append to this list in ways that might conflict or be incompatible with any such custom additions.

This section is non-normative.

Note that this change log only identifies substantive Changes since EPUB 3.2 — those that affect the conformance of Reading Systems or are similarly noteworthy.

For a list of all issues addressed during the revision, refer to the Working Group's issue tracker.

11-Feb-2022: Fix the contradictory support statements for Media Overlays. Support is recommended when rendering of prerecorded audio is supported, as in previous versions of EPUB 3. See issue 1991.
04-Feb-2022: Changed the informative security considerations for controlling network access, opening external links, and managing local storage to normative recommendations. See issue 1957.
04-Feb-2022: Expanded the section on security and privacy to include new sections on the threat model for Reading Systems and additional recommendations for ensuring security and privacy. See issue 1871, issue 1872, issue 1875 and issue 1876.
03-Feb-2022: Reduced the requirement to support deobfuscation to a recommendation. See issue 1873.
19-Jan-2022: Elevated default statements for the rendition:spread and rendition:orientation properties to requirements, to align with rendition:layout. See issue 1936.
08-Dec-2021: page-spread-center is now an alias for spread-none. See issue 1929.
10-Nov-2021: Proper definition of the Content URL and handling of relative URLs. See issue 1374 and issue 1888.
01-Nov-2021: Added a statement on the Reading System behavior when item references are repeated in the spine. See issue 1686.
25-Oct-2021: Clarify that reading systems must render non-linear resources when they are hyperlinked to by a user. See issue 1864.
27-Aug-2021: Added accessibility bullet to cover the need for zooming, particularly for fixed-layout documents. See issue 1776.
27-Aug-2021: Remove the recommendation to support text search and selection in SVGs. Replacing with a more general bullet on search accessibility to avoid defining requirements on reading system UI. See issue 1774.
09-July-2021: Restored the custom attributes section due to known usage but without reference to the attribute registry. See issue 1602.
09-July-2021: Removed recommendation to use the first language tag to determine an unspecified page progression direction. See issue 1482.
18-June-2021: Moved requirements for supporting SSML, PLS lexicons and CSS 3 Speech to the EPUB 3 Text-to-Speech Support note. See issue 1690.
20-May-2021: Providing a shared and unique origin for scripts is a requirements for Reading Systems that support scripting. See issue 1659.
20-May-2021: Removed requirement for reading systems to be conformant xml:base processors. See pull request 1678.
12-May-2021: Added additional requirement that Reading Systems ignore property values that expand to invalid URL strings and property values with empty references. See issue 1673.
12-May-2021: Changed all references to URIs/IRIs to URLs and references to RFCs 3986 and 3987 to the URL standard. See issue 808.
06-May-2021: Recommend that Reading Systems support references to HTML target elements and SVG fragment identifiers in textref attributes and the text element's src attribute. Support for other fragment identifier schemes is optional. See issue 1586.
23-Apr-2021: Clarified that Reading Systems should attempt to process invalid file and path names. See issue 1629
16-Apr-2021: Removed the section on custom attributes. See Working Group Resolution.
15-Apr-2021: Remove the reference to "presentation logic". See issue 1516 and Working Group Resolution.
12-Apr-2021: The section 16. epubReadingSystem Object has been marked as "at-risk". See Working Group resolution.
09-Apr-2021: Added a new section dedicated to accessibility and removed the outdated reference to the defunct appendix in the Accessibility specification. See issue 1608.
09-Apr-2021: Added section clarifying all the conformance requirements on establishing the primary language and base direction of Publication Resources. See pull request 1613.
01-Apr-2021: Added section clarifying how absolute URLs are created from relative in the Package Document. See pull request 1468.
30-Mar-2021: Restructured the document to remove the rendundant conformance sections and requirements that were only used as locators when this specification was combined with the authoring requirements. See pull request 1597 and pull request 1609.
23-Mar-2021: Added requirement to prevent top-level navigation to data URLs. See issue 1564.
23-Mar-2021: Changed "suppressing" of non-linear content to "skipping" when traversing the spine to clarify that the intention is not to remove all access to such content. See issue 1480.
10-Mar-2021: Changed restriction against using resources not listed in the Package Document to a recommendation not to use. See issue 810.
09-March-2021: the statement on unique identifiers has been set to non-normative, following the discussion and Working Group resolution. See issue 1310.
08-Mar-2021: Remove unnecessary requirement to assume default value for rendering metadata. See issue 1313.
05-Mar-2021: Added requirement for reading systems to collapse whitespace in DCMES and meta elements per the [Infra] specification definition. See issue 1295.
26-Feb-2021: Clarified the trimming of leading and trailing whitespace is to be done in accordance with the [Infra] specification definition, and removed the exception that meta element properties may define their own whitespace handling rules. See issue 1295.
19-Feb-2021: Updated the scripting security considerations so that the text recommends assigning a unique origin to each EPUB Publication for security rather than domain isolation at the Content Document level. See issue 1156.
15-Feb-2021: Clarified the requirements for presenting nav elements in the navigation document. The toc nav is now the only required element, the page-list nav is recommended when present, and all others are optional. See issue 975.
10-Feb-2021: A very first draft for 15. Security and Privacy has been added.
04-Feb-2021: Added explicit requirement that reading systems not use the language in dc:language elements as the language of resources.
02-Feb-2021: Added base direction determination rules for the dir attribute accounting for the addition of the new auto value. See issue 1491.
02-Feb-2021: Noted that language information in the new hreflang attribute on link elements is not authoritative. See issue 1488.
24-Dec-2020: The creation of a release identifier [EPUBPackages-32] is no longer defined. See issue 1440.
06-Nov-2020: Reading systems are now required to not resolve external identifiers in doctype declarations. See issue 1338.
05-Nov-2020: Generalized the restriction on custom attribute namespace URIs to exclude all of idpf.org and w3.org. See issue 1388.
30-Sept-2020: The structure of the EPUB core specifications has been simplified to ease understanding and access to information. This specification now consolidates the reading system requirements from the EPUB 3.2 specification together with the Packages, Content Documents, OCF and Media Overlays specifications. The EPUB 3.3 core specification now focuses only on authoring requirements.

[BIDI]: Unicode Bidirectional Algorithm. Mark Davis; Aharon Lanin; Andrew Glass. Unicode Consortium. 27 August 2021. Unicode Standard Annex #9. URL: https://www.unicode.org/reports/tr9/tr9-44.html
[CSSSnapshot]: CSS Snapshot. URL: https://www.w3.org/TR/CSS/
[DC11]: Dublin Core Metadata Element Set, Version 1.1. DCMI. 14 June 2012. DCMI Recommendation. URL: http://dublincore.org/documents/dces/
[DOM-Level-2-Style]: Document Object Model (DOM) Level 2 Style Specification. Chris Wilson; Philippe Le Hégaret. W3C. 3 November 2020. W3C Recommendation. URL: https://www.w3.org/TR/DOM-Level-2-Style/
[EPUB-33]: EPUB 3.3. Matt Garrish; Ivan Herman; Dave Cramer. W3C. 2 March 2022. W3C Working Draft. URL: https://www.w3.org/TR/epub-33/
[EPUBContentDocs-301]: EPUB Content Documents 3.0.1. Markus Gylling; William McCoy; Elika J. Etimad; Matt Garrish. IDPF. 26 June 2014. URL: http://idpf.org/epub/301/spec/epub-contentdocs-20140626.html
[HTML]: HTML Standard. Anne van Kesteren; Domenic Denicola; Ian Hickson; Philip Jägenstedt; Simon Pieters. WHATWG. Living Standard. URL: https://html.spec.whatwg.org/multipage/
[Infra]: Infra Standard. Anne van Kesteren; Domenic Denicola. WHATWG. Living Standard. URL: https://infra.spec.whatwg.org/
[MATHML3]: Mathematical Markup Language (MathML) Version 3.0 2nd Edition. David Carlisle; Patrick D F Ion; Robert R Miner. W3C. 10 April 2014. W3C Recommendation. URL: https://www.w3.org/TR/MathML3/
[Microdata]: HTML Microdata. Charles 'chaals' (McCathie) Nevile; Dan Brickley; Ian Hickson. W3C. 28 January 2021. W3C Working Group Note. URL: https://www.w3.org/TR/microdata/
[OpenType]: OpenType specification. Microsoft. URL: http://www.microsoft.com/typography/otspec/default.htm
[RDFA-CORE]: RDFa Core 1.1 - Third Edition. Ben Adida; Mark Birbeck; Shane McCarron; Ivan Herman et al. W3C. 17 March 2015. W3C Recommendation. URL: https://www.w3.org/TR/rdfa-core/
[RFC2119]: Key words for use in RFCs to Indicate Requirement Levels. S. Bradner. IETF. March 1997. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc2119
[RFC2397]: The "data" URL scheme. L. Masinter. IETF. August 1998. Proposed Standard. URL: https://www.rfc-editor.org/rfc/rfc2397
[RFC8174]: Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words. B. Leiba. IETF. May 2017. Best Current Practice. URL: https://www.rfc-editor.org/rfc/rfc8174
[SMIL3]: Synchronized Multimedia Integration Language (SMIL 3.0). Dick Bulterman. W3C. 1 December 2008. W3C Recommendation. URL: https://www.w3.org/TR/SMIL3/
[SVG]: SVG. W3C. URL: https://www.w3.org/TR/SVG/
[TrueType]: TrueType™ Reference Manual. Apple, Inc. URL: https://developer.apple.com/fonts/TrueType-Reference-Manual/
[URL]: URL Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://url.spec.whatwg.org/
[W3CProcess]: W3C Process Document. URL: https://www.w3.org/Consortium/Process/
[WAI-ARIA]: Accessible Rich Internet Applications (WAI-ARIA) 1.0. James Craig; Michael Cooper et al. W3C. 20 March 2014. W3C Recommendation. URL: https://www.w3.org/TR/wai-aria/
[WEBIDL]: Web IDL Standard. Edgar Chen; Timothy Gu. WHATWG. Living Standard. URL: https://webidl.spec.whatwg.org/
[WOFF]: WOFF File Format 1.0. Jonathan Kew; Tal Leming; Erik van Blokland. W3C. 13 December 2012. W3C Recommendation. URL: https://www.w3.org/TR/WOFF/
[WOFF2]: WOFF File Format 2.0. Vladimir Levantovsky; Raph Levien. W3C. 6 July 2021. W3C Recommendation. URL: https://www.w3.org/TR/WOFF2/
[XML]: Extensible Markup Language (XML) 1.0 (Fifth Edition). Tim Bray; Jean Paoli; Michael Sperberg-McQueen; Eve Maler; François Yergeau et al. W3C. 26 November 2008. W3C Recommendation. URL: https://www.w3.org/TR/xml/
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[CSS2]: Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification. Bert Bos; Tantek Çelik; Ian Hickson; Håkon Wium Lie. W3C. 7 June 2011. W3C Recommendation. URL: https://www.w3.org/TR/CSS21/
[DOM]: DOM Standard. Anne van Kesteren. WHATWG. Living Standard. URL: https://dom.spec.whatwg.org/
[EPUB-TTS-10]: EPUB 3 Text-to-Speech Enhancements 1.0. Matt Garrish. W3C. 8 December 2021. W3C Working Group Note. URL: https://www.w3.org/TR/epub-tts-10/
[EPUBPackages-32]: EPUB Packages 3.2. Matt Garrish; Dave Cramer. EPUB 3 Community Group. 08 May 2019. URL: https://www.w3.org/publishing/epub32/epub-packages.html
[H264]: H.264 : Advanced video coding for generic audiovisual services. 2017-04-13. URL: https://www.itu.int/ITU-T/recommendations/rec.aspx?rec=13189
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[RFC6386]: VP8 Data Format and Decoding Guide. J. Bankoski; J. Koleszar; L. Quillio; J. Salonen; P. Wilkins; Y. Xu. IETF. November 2011. Informational. URL: https://www.rfc-editor.org/rfc/rfc6386
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[ZIP]: .ZIP File Format Specification. 1 September 2012. Final. URL: https://www.pkware.com/documents/casestudies/APPNOTE.TXT

EPUB Reading Systems 3.3

Abstract

Status of This Document

1. Introduction

1.1 Overview

1.2 Terminology

1.3 Conformance

1.4 Relationship to Other Specifications

1.4.1 Relationship to HTML

1.4.2 Relationship to SVG

2. Publication Resource Processing

2.1 Foreign Resources

2.2 Remote Resources

2.3 Data URLs

2.4 Core Media Types

2.5 XML Processing

2.6 Internationalization

2.7 Network Access

2.8 External Links

3. Package Document Processing

3.1 Base Direction

3.2 Unique Identifier

3.3 Metadata

3.4 Manifest

3.5 Spine

3.5.1 Spine Overrides

3.6 Collections

4. Content Document Processing

4.1 XHTML Content Documents

4.1.1 HTML Extensions

4.1.1.1 ARIA

4.1.1.2 Structural Semantics

4.1.1.3 RDFa

4.1.1.4 Microdata

4.1.1.5 Content Switching (Deprecated)

4.1.1.6 The epub:trigger Element (Deprecated)

4.1.1.7 Custom Attributes

4.1.2 HTML Deviations and Constraints

4.1.2.1 MathML

4.1.2.2 Embedded SVG

4.1.2.2.1 Embedded SVG and CSS

4.1.2.3 Form Submission

4.2 SVG Content Documents

4.3 Cascading Style Sheets (CSS)

4.4 Scripting

4.4.1 Local Storage

4.4.2 Event Model

4.4.3 Security Considerations

5. Navigation Document Processing

6. Fixed-Layout Documents Processing

6.1 Fixed-Layout Properties

6.1.1 The rendition:layout Property

6.1.2 The rendition:orientation Property

6.1.3 The rendition:spread Property

6.1.4 The rendition:page-spread-* Properties

6.2 Initial Containing Block Dimensions

6.3 Viewport Rendering

7. Open Container Format Processing

7.1 OCF Abstract Container

7.1.1 URL of the Root Directory

7.1.2 File Names

7.1.3 META-INF Directory

7.2 OCF ZIP Container

7.3 Font Obfuscation

8. Media Overlays Processing

8.1 Loading the Media Overlay

8.2 Basic Playback

8.2.1 Timing and Synchronization

8.2.2 Rendering Audio

8.2.3 Rendering EPUB Content Document Elements

8.3 Interacting with the EPUB Content Document

8.3.1 Navigation

8.3.2 Embedded Audio and Video

8.3.3 Text-to-Speech

8.4 Skippability and Escapability

8.4.1 Skippability

8.4.2 Escapability

9. Processing Structural Semantics

10. Vocabulary Association Mechanisms

11. Package Rendering Metadata Processing

4.1.1.6 The `epub:trigger` Element (Deprecated)

6.1.1 The `rendition:layout` Property

6.1.2 The `rendition:orientation` Property

6.1.3 The `rendition:spread` Property

6.1.4 The `rendition:page-spread-*` Properties

7.1.3 `META-INF` Directory

11.1 The `rendition:flow` Property

11.2 The `rendition:align-x-center` Property