epub rs 34

EPUB Reading Systems 3.4 W3C Candidate Recommendation Draft 21 July 2026 More details about this document This version: https://www.w3.org/TR/2026/CRD-epub-rs-34-20260721/ Latest published version: https://www.w3.org/TR/epub-rs-34/ Latest editor's draft: https://w3c.github.io/epub-specs/epub34/rs/ History: https://www.w3.org/standards/history/epub-rs-34/ Commit history Test suite: https://w3c.github.io/epub-tests/index.html Implementation report: https://w3c.github.io/epub-specs/epub34/reports/ Editors: Matt Garrish ( DAISY Consortium ) Ivan Herman ( W3C ) Former editor: Dave Cramer ( W3C Invited Expert ) Feedback: GitHub w3c/epub-specs ( pull requests , new issue , open issues ) [email protected] with subject line [epub-rs-34] … message topic … ( archives ) Copyright © 1999-2026 International Digital Publishing Forum and World Wide Web Consortium . W3C ® liability , trademark and permissive document license rules apply. 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. Status of This Document This section describes the status of this document at the time of its publication. A list of current W3C publications and the latest revision of this technical report can be found in the W3C standards and drafts index . This document was published by the Publishing Maintenance Working Group as a Candidate Recommendation Draft using the Recommendation track . Publication as a Candidate Recommendation does not imply endorsement by W3C and its Members. A Candidate Recommendation Draft integrates changes from the previous Candidate Recommendation that the Working Group intends to include in a subsequent Candidate Recommendation Snapshot. This is a draft document and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to cite this document as other than a work in progress. Future updates to this upcoming Recommendation may incorporate new features . This document was produced by a group operating under the W3C Patent Policy . W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent that the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy . This document is governed by the 18 August 2025 W3C Process Document . Table of Contents 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. Reading system conformance 2.1 Requirements 2.2 Error handling 2.3 Error reporting 3. Publication resource processing 3.1 Core Media types 3.2 Foreign resources 3.3 Manifest fallbacks 3.4 Resource locations 3.5 Data URLs 3.6 File URLs 3.7 XML processing 3.8 Internationalization 3.9 Network access 3.10 External links 4. Open Container Format (OCF) processing 4.1 OCF abstract container 4.1.1 URL of the root directory 4.1.2 File names 4.1.3 META-INF directory 4.2 OCF ZIP container 4.3 Font obfuscation 5. Package document processing 5.1 Base direction 5.2 Unique identifier 5.3 Metadata 5.4 Manifest 5.5 Spine 5.5.1 Spine overrides 5.6 Collections 5.7 Legacy features 6. Layout processing 6.1 Layout types 6.1.1 Reflowable layouts 6.1.2 Fixed layouts 6.1.2.1 Pre-paginated layouts 6.1.2.1.1 Synthetic spread placement 6.1.2.2 Roll layouts 6.2 Custom properties 7. EPUB content document processing 7.1 XHTML content documents 7.1.1 HTML extensions 7.1.1.1 RDFa 7.1.1.2 Internationalization Tag Set (ITS) 7.1.1.3 Custom attributes 7.1.2 HTML deviations and constraints 7.1.2.1 Microdata 7.1.2.2 MathML 7.1.2.3 Embedded SVG 7.1.2.3.1 Embedded SVG and CSS 7.1.2.4 Form submission 7.2 SVG content documents 7.3 Fixed-layout documents 7.3.1 Initial containing block dimensions 7.3.2 The viewport meta tag 7.4 Cascading Style Sheets (CSS) 7.4.1 Author styling overrides 7.5 Scripting 7.5.1 Local storage 7.5.2 Event model 7.5.3 Security considerations 8. Navigation document processing 9. Aural rendering 9.1 Text-to-speech 9.2 Media overlays processing 9.2.1 Loading the media overlay 9.2.2 Basic playback 9.2.2.1 Timing and synchronization 9.2.2.2 Rendering audio 9.2.2.3 Rendering text-to-speech 9.2.2.4 Rendering EPUB content document elements 9.2.3 Navigating within EPUB content documents 9.2.4 Skippability and escapability 9.2.4.1 Skippability 9.2.4.2 Escapability 10. Processing structural semantics 11. Processing compact URLs 12. Backward compatibility 13. Forward compatibility 14. Accessibility 15. Security and privacy 15.1 Overview 15.2 Threat model 15.3 Recommendations A. Obsolete features A.1 Outdated features A.2 Deprecated features B. epubReadingSystem object B.1 Interface definition B.2 Description B.3 Methods B.3.1 hasFeature B.3.1.1 Description B.3.1.2 Features C. Index C.1 Terms defined by this specification C.2 Terms defined by reference D. Change log E. Acknowledgements F. References F.1 Normative references F.2 Informative references 1. Introduction 1.1 Overview 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-34 ], while this specification details the rendering requirements for them in EPUB reading system . 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. Therefore, there is no single set of rules that applies to all reading systems. Rather, this specification breaks down the rendering requirements based on a reading system's capabilities and the features it supports. Moreover, this specification allows for a great deal of flexibility for developers to create unique user interfaces, and requirements for things like metadata processing are intentionally minimal to allow for such flexibility. So, although this specification identifies the formal requirements for reading systems, it is not possible to understand this document in isolation. Developers also need to familiarize themselves with the full content structure of an EPUB publication in order 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. 1.2 Terminology This specification uses terminology defined in EPUB 3.4 [ epub-34 ]. It also defines the following term: 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. Note Only the first instance of a term in a section links to its definition. 1.3 Conformance 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. All caution boxes, explanatory statements in parentheses, and algorithm explanations are also non-normative . 1.4 Relationship to other specifications This section is non-normative. 1.4.1 Relationship to HTML 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 need to 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 , form submission , or the HTML DOM [ 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 form submission is not compulsory, a conformant reading system might not be a fully conformant HTML user agent. 1.4.2 Relationship to SVG 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 need to keep track of changes to the SVG standard to ensure that they keep their systems up to date. 2. Reading system conformance 2.1 Requirements Whether a reading system has to support a feature is mentioned at the beginning of its section. To be conformant with this specification, reading systems MUST support all required features as well as all applicable conditionally-required features (e.g., to support image rendering if the reading system has a viewport ) as defined in their respective sections. Note As a reading system is not necessarily a single application, but can exist as a distributed system, it is not always the case that reading system requirements will be met in the application that renders EPUB publications to users. An example is a reading system that solely interacts with a controlled content repository (e.g., a bookstore or library system). In this case, if a reading system developer can demonstrate that requirements are not applicable to the application (e.g., EPUB publications with duplicate itemref entries [ epub-34 ] cannot enter the system), the reading system as a whole is still considered conforming. When supporting recommended and optional features, reading systems MUST meet all normative requirements as defined in their respective sections. When reading system developers opt not to support a recommended or optional feature, it does not always mean none of the normative requirements of the section apply. In some cases, there might be alternative requirements when not implementing a feature (e.g., to process fallbacks when scripting is not supported). Reading systems MUST meet these alternative requirements when not supporting a feature. Note EPUB publications frequently contain additional information not specified in [ epub-34 ] (e.g., package document metadata). Reading systems can use this additional information for any purposes, such as to improve the user interface. 2.2 Error handling Reading systems do not have to load EPUB publications , or resources within them, when they violate content authoring or processing requirements. 2.3 Error reporting This section is non-normative. Although reading systems do not have to report errors encountered while processing and rendering EPUB publications (e.g., if the dimensions of a fixed-layout document have been inferred), they are strongly encouraged to provide a means of accessing this information. A comparable example are the developer tools that web browsers provide for debugging HTML pages and applications. Access to such processing information allows, for example, more efficient debugging of EPUB publications. For maximum use in debugging, it is advised that reading systems not only report issues they directly encounter but also issues reported by any applications they use (e.g., HTML, CSS, and JavaScript errors reported from a browser core used to render the content, or validation issues reported by EPUBCheck ). It is not expected that error reporting will be an intrusive experience that affects the general reading experience for users. Rather, reporting information could, for example, be selectively activated from a settings menu so that it does not bother users unnecessarily. 3. Publication resource processing Reading systems MUST process publication resources [ epub-34 ]. 3.1 Core Media types If a reading system has a viewport , it MUST support the image core media type resources [ epub-34 ]. If it has the capability to render prerecorded audio, it MUST support the audio core media type resources [ epub-34 ]. Note It is advised 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 can support any video codec, or none at all. Reading system developers have to take into consideration factors such as breadth of adoption, playback quality, and technology royalties when deciding which video formats to support. 3.2 Foreign resources Reading systems MAY support an arbitrary set of foreign resource types. 3.3 Manifest fallbacks For foreign content documents , if a reading system does not support the resource's MIME media type, it MUST traverse the resource's manifest fallback chain [ epub-34 ] until it identifies a supported publication resource to use in place of the unsupported resource. When manifest fallbacks [ epub-34 ] are provided for epub content documents , reading systems MAY choose from the available options to find the optimal version to render (e.g., by inspecting the properties attribute for each). If the reading system supports multiple publication resources in the fallback chain, it MAY select the resource to use based on the resource's properties attribute [ epub-34 ] values, otherwise it SHOULD honor the preferred fallback order [ epub-34 ]. If a reading system does not support any resource in the fallback chain, it MUST alert the user that it could not display the content. A reading system MUST terminate the fallback chain at the first reference to a manifest item it has already encountered. 3.4 Resource locations Reading systems SHOULD support remote resources , as defined in Resource locations [ epub-34 ]. To limit the risk of network attacks, reading systems SHOULD only load remote resources referenced via the https URI scheme [ rfc9110 ]. 3.5 Data URLs 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. 3.6 File URLs Reading Systems MUST prevent access to resources referenced via file URLs [ rfc8089 ]. 3.7 XML processing A reading system MUST use a non-validating XML processor [ xml ] that: does not read external DTD subsets [ xml ]; conforms to [ xml-names ]. 3.8 Internationalization As part of processing publication resources , a reading system has to process the attributes to set the language and the base directions in XHTML content documents or SVG content documents , as well as the xml:lang attribute for all XML documents (e.g., the package document and media overlay documents ). Furthermore, a reading system MUST also process the dir attribute [ epub-34 ] for the package document , where the base direction specified by dir applies to the element where it is specified, and to all elements in its content unless overridden with another instance of dir . (See also 5.1 Base direction for further details.) 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 [ epub-34 ]). Refer to a resource's formal specification for more information about to handle the absence of explicit language or direction information. 3.9 Network access Reading systems MAY support network access to retrieve 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 that most reading systems 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 allow network access, it is RECOMMENDED both that they: notify users when network activity occurs; and let users block access to the network (e.g., disable network access for the reading system globally or for a particular EPUB publication). 3.10 External links When a link has an http or https scheme [ url ], reading systems: SHOULD obtain the user's consent before opening the link. SHOULD open the link in a new browser instance to ensure that the browser's security and privacy controls are available to users. For links that have schemes that require a helper application to load, reading systems SHOULD obtain the user's consent and follow platform guidance for opening the helper application. Note External links are not only found in EPUB content documents . A reading system might provide access to external linked records [ epub-34 ] in the package document metadata, for example. Note For more information about security and privacy issues with external links, see 15.2 Threat model . 4. Open Container Format (OCF) processing Reading systems MUST process the EPUB container [ epub-34 ]. Note An application that processes EPUB 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. 4.1 OCF abstract container 4.1.1 URL of the root directory 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 [ url ] 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 [ html ] 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 properties of the container root URL are such that a conforming reading system will parse any relative URL string to a content URL . In other words, relative links do not "leak" outside the container content, which is an important feature for security. In practice, the container root URL 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 EPUB/package.opf http://localhost:49152/EPUB/package.opf EPUB content document HTML/file name.xhtml http://localhost:49152/HTML/file%20name.xhtml URL string (found for example in the package document) content URL ../HTML/file%20name.xhtml http://localhost:49152/HTML/file%20name.xhtml /Media/img.png http://localhost:49152/Media/img.png ../../../Media/img.png http://localhost:49152/Media/img.png Note that the last two links are disallowed in an EPUB publications to ensure better interoperability with non-conforming or legacy reading systems and toolchains. 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 have to 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-34 ]. 4.1.2 File names Although [ epub-34 ] defines file name and file path restrictions for maximum interoperability, reading systems SHOULD attempt to process file names and paths that do not adhere to these requirements. Invalid file names and paths might 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 handle this incompatibility. 4.1.3 META-INF directory Container file ( container.xml ) A reading system MUST , by default, use the package document referenced the from first rootfile element [ epub-34 ] 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 [ epub-34 ] with unrecognized root elements. Manifest file ( manifest.xml ) Reading systems MUST NOT use ancillary manifest information contained in the ZIP archive or in the manifest.xml file [ epub-34 ] for processing an EPUB publication . Signature file ( signatures.xml ) Before computing the digest used to validate the signature in the signatures.xml file [ epub-34 ], reading systems MUST decrypt any data encrypted after signing — data encrypted before signing MUST NOT be decrypted. Refer to Decryption Transform for XML Signature [ xmlenc-decrypt ] for more information about identifying data encrypted after signing. Other files Reading systems MUST NOT fail when encountering configuration files in the META-INF directory not listed in Reserved files [ epub-34 ]. 4.2 OCF ZIP container A reading system: MUST treat any OCF ZIP container that splits the content into segments [ zip ] as in error. MUST treat any OCF ZIP container that uses compression techniques other than Deflate [ rfc1951 ] as in error. MUST support the ZIP64 extensions defined as "Version 1" [ zip ]. MUST treat an OCF ZIP container that uses [ 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 [ zip ] as being in error. MUST treat compression method field values other than 0 or 8 in the local file header table [ zip ] as being in error. MUST treat OCF ZIP containers with an Archive decryption header or an Archive extra data record [ zip ] as being in error. 4.3 Font obfuscation Caution NIST is advising that use of the SHA-1 algorithm [ fips-180-4 ] be phased out by the end of 2030. The Publishing Maintenance Working Group does not intend to support font obfuscation in EPUB publications past that date due to its reliance on SHA-1 — the feature is now marked as outdated to not invalidate existing EPUB publications. Reading system developers will have to consider continuing to support deobfuscation to fully support all EPUB publications that have used the feature. As CSS provides font fallbacks, however, ending support will only affect the appearance of some text content. Reading systems SHOULD support deobfuscation of fonts as defined in Font obfuscation [ epub-33 ]. To restore the original data, reading systems can simply reverse the process: the source file becomes the obfuscated data, and the destination file contains the raw data. Note EPUB 3 allowed font obfuscation 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 might return a valid font. Reading systems do not have to support this method of retrieval, but developers need to consider that it will likely be encountered if they support EPUB 3 content generally. 5. Package document processing Reading systems MUST process the package document [ epub-34 ]. 5.1 Base direction If the dir attribute [ epub-34 ] 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 ]. Note Reading system with an international audience, or that claim support for international content, are strongly advised to implement this feature when exposing that metadata to users. Ignoring the directionality of text can cause readability issues. 5.2 Unique identifier 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, could require inspecting other metadata, such as their last modification dates, titles, and authors. 5.3 Metadata Whitespace Reading systems MUST strip and collapse ASCII whitespace [ infra ] from Dublin Core [ dcterms ] and meta element values [ epub-34 ] before processing. The dc:identifier element To determine whether the value of a dc:identifier element [ epub-34 ] conforms to an established system or has been granted by an issuing authority, reading systems SHOULD check for an identifier-type property [ epub-34 ]. The dc:title element Reading systems MUST recognize the first dc:title element [ epub-34 ] 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 dc:title elements. The dc:language element The language(s) of the EPUB publication specified in dc: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 3.8 Internationalization for more details on how to determine the language of a publication resource . The dc:creator element When determining display priority, reading systems MUST use the document order of dc:creator elements [ epub-34 ] 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 Reading systems SHOULD ignore all meta elements whose property attributes [ epub-34 ] define expressions they do not recognize. A reading system MUST NOT fail when encountering unknown expressions. If a reading system does not recognize the scheme attribute [ epub-34 ] value, it SHOULD treat the value of the element as a string. The link element Retrieval and support of linked resources is OPTIONAL . The language identified in an hreflang [ EPUB-34 ] attribute is purely advisory. The language information expressed in the resource determines its language for processing and rendering purposes, as defined in the internationalization requirements . 5.4 Manifest Reading systems MUST ignore values of the properties attribute [ epub-34 ] they do not recognize. Reading systems SHOULD NOT use linked 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.). 5.5 Spine Reading systems MUST provide a means of rendering an EPUB publication in the order defined in the spine element [ epub-34 ], which includes: recognizing the first primary (linear) itemref [ epub-34 ] as the beginning of the default reading order; and, rendering successive primary items in the order given in the spine . Note Refer to 3.3 Manifest fallbacks for more information on handling manifest fallbacks. When a user traverses the default reading order defined in the spine element, a reading system MAY automatically skip non-linear itemref elements [ epub-34 ]. 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 skip non-linear content by default or not. If the page-progression-direction attribute [ epub-34 ] is not set, the reading system MUST assume the value of default . When page-progression-direction value is default , the reading system can choose the rendering direction. If the page-progression-direction attribute has a value other than default , the reading system MUST ignore any directionality computed from pre-paginated XHTML content documents . Reading systems MUST ignore all values expressed in spine itemref properties attributes [ EPUB-34 ] that they do not recognize. Reading systems MUST NOT skip spine references to duplicate manifest items [ EPUB-34 ] when rendering the linear reading order. The reading system MUST treat these as distinct items for user interface (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 navigate to the first occurrence of the document in the linear reading order. 5.5.1 Spine overrides When a spine itemref element's properties attribute overrides a global rendering property [ epub-34 ], reading systems MUST follow the requirements for the override's global value to display that spine item. For example, a spine item that contains the layout-pre-paginated override [ epub-34 ] 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 use only the first value. 5.6 Collections In the context of this specification, support for collections [ epub-34 ] in reading systems is OPTIONAL . Reading systems MUST ignore collection elements that define unrecognized roles. 5.7 Legacy features Reading systems MUST NOT support legacy features in content that conforms to this version of EPUB [ epub-34 ]. 6. Layout processing 6.1 Layout types 6.1.1 Reflowable layouts Reading systems MUST support reflowable layouts. The default value reflowable MUST be assumed by EPUB reading systems as the global value if no meta element carrying the rendition:layout property occurs in the package document metadata [ epub-34 ]. Reading systems MAY apply dynamic pagination when rendering reflowable EPUB content documents . 6.1.2 Fixed layouts 6.1.2.1 Pre-paginated layouts Reading systems MUST support pre-paginated layouts. Reading systems MUST produce exactly one page per spine itemref [ epub-34 ] when rendering. The default expectation for rendering pre-paginated layouts 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. When rendering synthetic spreads , reading systems MUST NOT include space between the adjacent content slots. 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. 6.1.2.1.1 Synthetic spread placement The spread placement spine overrides are interpreted as follows: When either the rendition:page-spread-left property or page-spread-left property [ epub-34 ] is specified, it indicates that the given spine item SHOULD be rendered in the left-hand slot of a spread. When either the rendition:page-spread-right property or page-spread-right property [ epub-34 ] is specified, it indicates that the given spine item SHOULD be rendered in the right-hand slot of a spread. When the rendition:page-spread-center property [ epub-34 ] is specified, it indicates that synthetic spreads SHOULD be disabled and the spine item SHOULD be rendered centered in the viewport. The rendition:page-spread-left , rendition:page-spread-right , rendition:page-spread-center properties (and their unprefixed equivalents) apply only to pre-paginated content, and only when the reading system is creating synthetic spreads . The rendition:page-spread-* properties and their unprefixed equivalents MUST take precedence over whatever value of the page-break-before property [ css2 ] has been set for an XHTML content document . 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. If the first itemref element [ epub-34 ] in the spine does not specify a spread placement position, it SHOULD either be presented by itself (i.e., as though the rendition:page-spread-center property has been set) or placed alone in one of the two available spread slots (i.e., with the other slot empty). Which slot to use SHOULD be obtained from the page-progression-direction attribute [ epub-34 ] value. For example, in an EPUB publication with a left-to-right page progression direction, the left-hand slot will be unpopulated. Note If a reading system does not render non-linear [ epub-34 ] content by default, the first spine item to place on its own becomes the first linear item in the spine. 6.1.2.2 Roll layouts Reading systems SHOULD support the rendering of roll layouts. For roll layouts [ epub-34 ], reading systems MUST fit the width of each spine itemref to the viewport and display them in a continuous roll with no visible gap. Reading systems MUST ignore any layout overrides [ epub-34 ] when an EPUB publication specifies to use a roll layout. Note Prior to the introduction of roll layouts, some Japanese publishers combined a pre-paginated layout with a rendition:flow property [ epub-33 ] set to the value " scrolled-continuous " to create rolls. If reading systems developers intend to support this content, they will need to treat the combination as synonymous with a roll layout being declared. 6.2 Custom properties Reading systems MAY support custom properties provided they do not introduce expressions that conflict behaviorally with the properties defined in the Package rendering vocabulary [ epub-34 ]. 7. EPUB content document processing The definition of EPUB content documents [ epub-34 ] includes various authoring restrictions to optimize the cross-compatibility of content (e.g., prohibiting CSS for setting language and direction [ epub-34 ]). Unless stated otherwise in this specification, reading systems MAY support these restricted features. 7.1 XHTML content documents Reading systems MUST process XHTML content documents [ epub-34 ]. 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. 7.1.1 HTML extensions 7.1.1.1 RDFa Reading system support for the attribute processing model [ rdfa-core ] is OPTIONAL . 7.1.1.2 Internationalization Tag Set (ITS) Reading system support for processing ITS markup [ its20 ] is OPTIONAL . 7.1.1.3 Custom attributes Reading systems MAY support custom attributes provided the attributes do not modify the requirements of this specification. 7.1.2 HTML deviations and constraints 7.1.2.1 Microdata Reading system support for the attribute processing model is OPTIONAL , as is the conversion to JSON [ html ]. 7.1.2.2 MathML To support MathML [ mathml3 ] 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 [ mathml3 ]. Note Reading systems can opt for third-party libraries such as MathJax to provide MathML rendering. 7.1.2.3 Embedded SVG Reading systems MUST process SVG embedded in XHTML content documents as defined in 7.2 SVG content documents . 7.1.2.3.1 Embedded SVG and CSS 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. 7.1.2.4 Form submission Reading system support for the submission of [ html ] form elements is OPTIONAL . A reading system might, for example, prevent form submissions by limiting access to networking. 7.2 SVG content documents Reading systems MUST process SVG content documents [ epub-34 ]. 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 7.5 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 7.1.2.3.1 Embedded SVG and CSS . 7.3 Fixed-layout documents Reading systems MUST support the rendering of EPUB content documents in spine items whose layout is designated as pre-paginated or roll . They MAY support the rendering of image foreign resources in spine items whose layout is designated as pre-paginated or roll . Note This requirement does not require reading systems to support roll layouts. Only if the layout is supported do reading systems have to support the rendering of fixed-layout EPUB content documents. 7.3.1 Initial containing block dimensions XHTML Reading systems MUST create the initial containing block (ICB) using the width and height expressions declared in the viewport meta tag for XHTML content documents , as defined in Expressing in HTML [ epub-34 ]. They MUST clip content positioned outside of the ICB. If the width or height values in the viewport meta tag contain a non-numeric character but start with a number (e.g., the value includes a unit of length declaration such as "500px"), the number prefix SHOULD be used as the pixel value, otherwise the value MUST be treated as invalid. Reading systems SHOULD attempt to extract width and height values from a viewport meta tag even if its syntax is invalid. Reading systems MUST use the first declaration of a width and height property in a viewport meta tag (i.e., ignore repeated declarations). If the viewport meta does not contain a width or a height value, or if these values are invalid, reading systems MAY supply the values. For example, a reading system might: consider these as having the values device-width and device-height , respectively; or look for the previous content document in the spine , if applicable, and use the ICB value defined there; or consider the content document as erroneous XHTML content altogether. If an XHTML content document contains more than one viewport meta tags, reading systems MUST use the first in document order to obtain the height and width dimensions. Subsequent declarations MUST be ignored. 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 create the initial containing block (ICB) using the dimensions as defined in Expressing the ICB in SVG [ epub-34 ] to render SVG content documents . Note When the ICB aspect ratio does not match the aspect ratio of the reading system content display area , it is advised to follow the rules for the viewBox and the preserveAspectRatio attributes, as defined in [ SVG ]. Image foreign resources If a reading system supports image foreign resources in the spine, it will have to create the initial containing block using the dimensions as defined in the width and height metadata for the images. Refer to the documentation for each supported image format for more information on how to obtain this metadata. 7.3.2 The viewport meta tag Except when obtaining the initial containing block dimensions for XHTML fixed-layout documents , reading systems MUST ignore rendering instructions in viewport meta declarations . This restriction applies to both fixed-layout and reflowable XHTML content documents . 7.4 Cascading Style Sheets (CSS) If a reading system has a viewport , it MUST support the visual rendering of XHTML content documents via CSS [ epub-34 ]. 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 [ css-fonts-4 ]. SHOULD support all prefixed properties defined in CSS Style Sheets — Prefixed properties [ epub-33 ]. SHOULD apply style sheets referenced from EPUB content documents as written. SHOULD support the [ html ] suggested default rendering in their user agent style sheet(s). Note Reading system developers are expected to publicly document their user agent style sheet(s) and how they interact with the styling set in EPUB publications. 7.4.1 Author styling overrides A reading system MAY override parts of an EPUB publication's styling (e.g., for ergonomic or user interface reasons). Morevoer, they MAY allow users to set default themes and style preferences (e.g., for more accessible reading) that further alter the original design of a publication. 7.5 Scripting Reading systems SHOULD support scripting [ epub-34 ]. Reading system support for scripting depends on its usage context: Reading systems SHOULD support container-constrained scripting [ epub-34 ] in reflowable EPUB content documents . Reading systems SHOULD support spine-level scripting [ epub-34 ] in XHTML and SVG fixed-layout documents [ epub-34 ]. Reading systems SHOULD support spine-level scripting in reflowable EPUBs where content is rendered with scrollable overflow. Similarly, if a reading system supports spine-level scripting in reflowable EPUBs, it MUST give users the option to scroll overflow content and SHOULD provide the option to view the publication as one continuous scroll from spine item to spine item. 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 might 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 [ html ], shared by all spine-level scripts of the EPUB publication . That origin [ html ] 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 host EPUB 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, a reading system might impose restrictions on modifications. See 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 B. epubReadingSystem object . It also MUST support the dom-manipulation and layout-change features defined in B.3.1.2 Features in container-constrained scripting contexts. If a reading system does not support scripting, it MUST process fallbacks for scripted content as defined in Fallbacks for scripted content documents [ epub-34 ]. 7.5.1 Local storage Reading systems MAY block scripts from saving persistent data through cookies and web storage [ html ]. Reading systems that allow local storage [ html ] SHOULD provide methods for users to inspect or delete that data. 7.5.2 Event model 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 have to 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 ought to be able to cancel the default action of any event, some events either might not be passed through or might not be cancelable. 7.5.3 Security considerations This section is non-normative. Reading system developers who also support scripting need to 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 have to take into consideration the same kinds of issues encountered in web contexts. Each reading system has to establish if it can trust the scripts in a particular document. It is advised to treat all scripts as untrusted (and potentially malicious) and to examine and protect against all vectors of attack, in particular: 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 EPUB 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 [ html ] allocated to each EPUB publication (see 4.1.1 URL of the root directory ). Assigning a unique origin ensures that spine-level scripts [ epub-34 ] are isolated from other EPUB publications, and limits access to cookies [ html ], web storage [ html ], etc. Examples of web APIs that are tied to the concept of "origin" include web storage [ html ] 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") might maintain the publication's unique origin when the publication is removed and subsequently re-imported into the content library. Conversely, reading systems might create a new unique origin for every newly added publication. This specification also recommends that container-constrained scripts [ epub-34 ] not be allowed to modify the DOM of the host EPUB content document and/or manipulate the containing rectangle (see 7.5 Scripting ). Note that compliance with these recommendations does not guarantee protection from the possible attacks listed above; developers have to examine each potential vulnerability within the context of their reading system. 8. Navigation document processing Reading systems MUST process EPUB navigation documents [ epub-34 ]. To process the EPUB navigation document, a reading system: MUST provide users access to the links and headings in the toc nav element [ epub-34 ]. SHOULD , when generating non-HTML based navigation widgets, replace unsupported non-text elements in headings and labels with their alternative text [ epub-34 ]. If both alt and title attributes are present on an element, preference SHOULD be given to the alt attribute. If neither attribute is present, reading systems MAY provide their own text or ignore the element. SHOULD provide a method to navigate to the listed page breaks when a page-list nav element [ epub-34 ] is present. MAY provide users access to the links in the landmarks nav element [ epub-34 ] and MAY use the links to enable functionality in the application. MAY provide access to nav elements that do not specify an epub:type attribute or that declare an unknown value [ epub-34 ]. MUST relocate the current reading position to the destination identified by an activated link when the link is to a publication resource . MUST NOT show list item numbering on nav elements when presenting them outside of the spine, regardless of support for CSS, as the default display style of list items is equivalent to the list-style: none property [ css2 ]. MUST ignore markup and styling intended to hide elements of the navigation document from rendering in the spine (e.g., the [ html ] hidden attribute and CSS display property [ css2 ]). Reading systems MUST honor the above requirements irrespective of whether the EPUB navigation document is also included in the spine . 9. Aural rendering 9.1 Text-to-speech Reading system support for text-to-speech playback is OPTIONAL . If a reading system has audio playback capabilities, it is strongly encouraged to provide text-to-speech playback functionality to improve the accessibility. When including text-to-speech playback, reading systems SHOULD support the following features to provide a playback experience similar to that defined for media overlays : sequential reading of the elements in the [ html ] body element of an EPUB content document ; uninterrupted playback across spine items; uninterrupted reading when a user traverses a hyperlink to another location within the EPUB publication ; skippability and escapability based on the markup and semantics available in the EPUB content document; highlighting of the word(s) being read; control over the playback rate. Note The Working Group expects to provide guidance on content-level issues with text-to-speech playback in a future note. This will include language- and script-specific issues, such as those currently addressed in [ ruby-tts-req ]. Note Possible authoring technologies for enhancing the quality of text-to-speech playback are detailed in [ epub-tts-10 ], but these technologies are currently not broadly supported (or supported at all). If these or replacement technologies find support in the future, they will be included in this specification. 9.2 Media overlays processing Reading systems with the capability to render prerecorded audio SHOULD support media overlays [ epub-34 ]. If a reading system does not support media overlays, it MUST ignore both: the media-overlay attribute on manifest item elements [ epub-34 ]; and item elements where the media-type attribute value equals application/smil+xml . 9.2.1 Loading the media overlay When a reading system loads a package document , it MUST refer to the manifest item elements' [ epub-34 ] 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 could 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. 9.2.2 Basic playback 9.2.2.1 Timing and synchronization Reading systems MUST render immediate children of the body element [ epub-34 ] in a sequence. A seq element's [ epub-34 ] children MUST be rendered in sequence, and playback completes when the last child finishes playing. Reading system MUST render a par element's [ epub-34 ] 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. 9.2.2.2 Rendering audio When presented with a media overlay audio element [ EPUB-34 ], reading systems MUST play the audio resource referenced by the src attribute, starting at the clip offset time given by the clipBegin attribute and ending at the clip offset time given by the clipEnd attribute [ epub-34 ]. In addition: If a clipBegin attribute is not set, reading systems MUST assume the value " 0 ". If a clipEnd attribute is not set, reading systems MUST assume the value to be the full duration of the physical media. If the value of 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. 9.2.2.3 Rendering text-to-speech When a media overlay text element with no audio [ epub-34 ] 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). This also means that reading systems need to treat the duration property values set in the package document as approximative when making use of them. 9.2.2.4 Rendering EPUB content document elements When presented with a media overlay text element [ epub-34 ] whose src attribute contains a URL-fragment string referencing a specific part of an EPUB content document, reading systems SHOULD ensure the referenced portion is visible in the viewport . In addition to [ html ] element ID references and SVG Fragment Identifiers [ svg ], 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-34 ] 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-34 ]. 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-34 ] does not reference an element is also implementation-specific. 9.2.3 Navigating within EPUB content documents 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 of the navigation point target. Note A media overlay document can be associated directly with an EPUB 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-34 ] for more information. Note Media overlay document elements can be associated with EPUB content document structures such as tables. Reading systems need to 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. 9.2.4 Skippability and escapability 9.2.4.1 Skippability Reading systems SHOULD use the semantic information provided by media overlay elements' epub:type attribute to offer users the option of skipping content. When skipping of content is enabled, reading systems MUST suppress playback of any par and seq elements whose epub:type attribute contains a semantic that matches a skippable structure. 9.2.4.2 Escapability While playing media overlays, reading systems SHOULD offer users the option to leave ("escape") escapable structures [ epub-34 ], which are determined by the presence of an epub:type attribute [ epub-34 ] with a value from the escapable structures list. If a user opts to escape from an escapable structure, then the reading system MUST continue playback with the next sequential element after the structure. 10. Processing structural semantics Reading systems MAY support structural semantics [ epub-34 ] in EPUB content documents . When processing the epub:type attribute, a reading system: MAY obtain an expanded URL for each value. MUST ignore the terms if used on an element whose usage is not allowed in the definition of epub:type [ epub-34 ]. MAY associate behaviors with none, some, or all of the terms defined in the default vocabulary [ epub-34 ]. MAY associate behaviors with terms from other vocabularies. 11. Processing compact URLs Reading systems MAY support the vocabulary association mechanisms for processing compact URLs [ epub-34 ]. This section defines an algorithm for obtaining an expanded URL from a compact URL. It applies only to reading systems that support the [ epub-34 ] vocabulary association mechanisms. Reading systems that do not support the [ epub-34 ] vocabulary association mechanisms MAY process compact URLs as plain string values [ infra ]. They do not have to support the expansion of these values to URLs or add reading system behaviors based on their presence. The algorithm describes the process using the terminology and data types defined in [ infra ], and, if successful, results in a string value with the expanded URL being returned. A null value is returned for invalid properties. This algorithm takes the following arguments: value : the compact URL to process. attr : the name of the attribute that contains value . elem : the name of the element that attr is attached to. doc : the document being processed (e.g., package document , media overlay document , epub content document ). To obtain an expanded URL , apply the following steps: Let baseURL , expandedURL , prefix , and reference be empty strings . Explanation In this algorithm: baseURL will hold the base URL associated with the value, whether this URL is assigned in a prefix attribute, corresponds to a default vocabulary, or comes from a reserved prefix. expandedURL will hold the final URL that results from the concatenation of the base URL and reference. prefix will hold the compact URL's prefix [ epub-34 ] (i.e., the value before the colon). Properties from a default vocabulary will not have a prefix, but all others will. reference will hold the compact URL's reference [ epub-34 ] (i.e., the value after the prefix). All valid properties require a reference. Obtain the values of prefix and reference as follows: If value does not contain a colon (U+003A), set reference to value . Explanation If a compact URL value does not have a colon, it does not have a prefix. In this case, the value is drawn from the default vocabulary as described in the next step of the algorithm. Otherwise, if value begins with a colon, the value is invalid. Return null . Explanation A colon at the beginning of the value is invalid as it indicates the prefix is not set. Otherwise, split value on the first colon and set prefix to the string before the colon and set reference to the string after the colon. If reference is not a valid path-relative-scheme-less-URL string , as the compact URL definition [ epub-34 ] requires, the value is invalid. Return null . Explanation The path-relative-scheme-less-URL string definition has a number of restrictions that apply to the reference whether it has a prefix or not. For example, the reference is not allowed to begin with a URL-scheme string followed by a colon. This restriction means that the following is not a valid property value as the second colon could represent a scheme: foo:bar:baz/qux . There are also restrictions on what characters have to be percent encoded. Obtain the value of baseURL as follows: If prefix is an empty string: If attr is epub:type [ epub-34 ], set baseURL to http://idpf.org/epub/vocab/structure/# Otherwise, if elem is the package document meta element [ epub-34 ] and attr is scheme , the property value is invalid. Return null . Otherwise, if elem is the package document meta element [ epub-34 ], set baseURL to http://idpf.org/epub/vocab/package/meta/# Otherwise, if elem is the package document link element [ epub-34 ], set baseURL to http://idpf.org/epub/vocab/package/link/# Otherwise, if elem is the package document item element [ epub-34 ], set baseURL to http://idpf.org/epub/vocab/package/item/# Otherwise, if elem is the package document itemref element [ epub-34 ], set baseURL to http://idpf.org/epub/vocab/package/itemref/# Explanation If the compact URL does not have a prefix, then this step assigns the default vocabulary URL to use based on the attribute or element to which it belongs. When an element in the package document has more than one attribute that takes a compact URL data type, those attributes share the same default vocabulary. For this reason, it is not necessary to check both the element and attribute to determine what URL to assign. The one attribute in EPUB 3 that does not have a default vocabulary is the scheme attribute [ epub-34 ]. A scheme value without a prefix is invalid. Otherwise: If a prefix attribute is declared on the document element of doc , and the attribute contains a prefix that matches prefix , set baseURL to the URL associated with the prefix declaration. Otherwise, if prefix matches a reserved prefix [ epub-34 ] for doc , set baseURL to the URL associated with the reserved prefix. Explanation When a compact URL has a prefix, the first place to check for its base URL is in a prefix attribute declaration on the document element. If the author has not assigned a URL for the prefix, the next step is to check if the prefix matches one of the reserved prefixes. Note that author-assigned prefix URLs override the reserved prefix URLs. If baseURL is an empty string, the property value is invalid. Return null . Explanation If the base URL is empty at the end of this processing step, then either the prefix is not mapped or reserved, or the element or attribute being processed does not accept compact URL data types as defined in [ epub-34 ]. Without a base URL, it is impossible to generate an expanded URL, so the value is invalid. Set expandedURL to the concatenated values of baseURL and reference . Do not add a separator when concatenating the values. If expandedURL is not a valid URL string , the value is invalid. Return null . Otherwise, return expandedURL . Explanation Although an expanded value might be obtained, it is not necessarily the case that it is a valid URL. This is only likely to occur when an invalid base URL is assigned to a prefix. Note Reading systems do not have to parse the resulting URL [ url ] or attempt to dereference the resulting expanded URL. Expanding a compact URL to a full URL only ensures a reading system has encountered the value it expects (i.e., because different EPUB publications might assign the same prefix to different URLs, two values might appear the same until expanded). 12. Backward compatibility Reading systems MUST attempt to process an EPUB publication whose package document version attribute [ epub-34 ] is less than " 3.0 ". EPUB publications with older version numbers will not always render exactly as intended unless processed according to their respective specifications. Reading systems SHOULD support such EPUB publications as defined by those specifications. 13. Forward compatibility Reading systems SHOULD attempt to process an EPUB publication whose package document version attribute [ epub-34 ] is greater than " 3.0 ". 14. Accessibility This section is non-normative. Although the primary focus of this specification is on how to process and render EPUB publications , it does not mandate specific user interfaces that all reading systems have to offer. This does not mean that there are not common accessibility issues that all reading systems developers need to be aware of or seek to avoid in their applications. The W3C 's User Agent Accessibility Guidelines [ uaag20 ] provides many useful practices developers can apply to improve their reading systems as many browser accessibility issues have parallels in EPUB-specific user agents. The following list outlines some additional EPUB-specific 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 content. 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 that 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 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 expressed in the source. Table of Contents — Ensure that users can access the link text, including text alternatives for visual content , and that users can activate the table of contents links using different inputs. The DAISY Consortium maintains an accessibility test suite to aid in evaluating these issues and more. 15. Security and privacy 15.1 Overview 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-34 ]. 15.2 Threat model This section is non-normative. The greatest threats to users come from the content they read [ epub-34 ], and the first line of defense 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 need to 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 could attempt unauthorized collection of user data. More detailed discussion of these issues and how to mitigate them is provided in 7.5.3 Security considerations . Malicious content EPUB publications could contain resources designed to exploit security flaws in reading systems or the operating systems they run on. Attackers could also try to gain access to remote resources using file indirection techniques, such as symbolic links or file aliases. The lack of a standard method of signing EPUB publications means that reading systems cannot always verify whether the content has been tampered with between authoring and loading in the device. 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 could be compromised. Calls to remote resources can also be used to track information about users (e.g., through server logs). It is advised to limit the information exposed through HTTP requests to only what is essential to obtain the resource. The origin of an EPUB is both specific to each reading system implementation and not known outside that implementation. Consequently, even when remote resources are hosted on a trusted web server, 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 could be used to trick unwitting users into opening malicious resources on the web designed to exploit the reading system or operating system. Likewise, it is advised to 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 7.5.3 Security considerations . Collection of user data Collecting information about the user and their reading habits without obtaining their permission can violate their privacy, 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 ] and [ 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. 15.3 Recommendations 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 behavior 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 SHOULD treat that data as sensitive and not allow access to it by third parties. It is understood that the collection of some user data is often necessary 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, the reading system SHOULD identify the data being collected, how it is used, and allow for user opt-outs (retailers could choose to inform users by other means, such as when a user creates an account on their web site). Anonymization of any collected data is RECOMMENDED for the privacy and the security of the user and reading system. It is also understood that user data could be necessary or helpful for some reading system affordances. In these cases, anonymization is also RECOMMENDED . Reading systems SHOULD also 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 — also need to 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. Reading systems SHOULD treat content from any new source, including from an integrated bookstore, as insecure (e.g., prompt users to allow scripting and network access the first time the source is accessed). If a reading system allows users to load their own content (e.g., through the process of "sideloading"), each instance SHOULD be treated as insecure. When processing XML documents, a reading system SHOULD NOT resolve external identifiers in DOCTYPE, ENTITY and NOTATION declarations [ xml ]. Reading systems SHOULD also consider security risks related to internal or external XML entities like, for example, DoS attacks also known as "Billion laughs attacks" or "XML external entity attacks" . Note Additional security recommendations for external links, network access and scripting are available in 3.10 External links , 3.9 Network access , and 7.5.3 Security considerations , respectively. A. Obsolete features A.1 Outdated features Reading systems MAY support outdated features [ epub-34 ] with the exception of the legacy features . Legacy features are only allowed for compatibility with EPUB 2 reading systems and MUST be ignored by EPUB 3 reading systems. A.2 Deprecated features Reading systems MAY support deprecated authoring features [ epub-34 ]. In addition, the following reading system features are now deprecated: Media overlays processing Automatic playback of embedded audio and video [ epubmediaoverlays-32 ] epubReadingSystem object layoutStyle property [ epubcontentdocs-301 ] name and version properties [ epubcontentdocs-32 ] Note Developers need to consider the unlikelihood of encountering content with deprecated features before adding new support for them. B. epubReadingSystem object 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. B.1 Interface definition This specification extends the Navigator object [ html ] as follows. WebIDL [ Exposed =(Window) ] interface EpubReadingSystem { 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 ]. Therefore, reading systems do not have to expose the epubReadingSystem object in the scripting context of Workers, so its presence cannot be relied on. B.2 Description The Navigator.epubReadingSystem object provides an interface through which a scripted content document can query information about a user's reading system. 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-34 ]. Reading systems MUST ensure that the epubReadingSystem object is available no later than when the DOMContentLoaded event is triggered [ html ]. Note Reading system implementations can create cloned instances of the epubReadingSystem object in scripted content documents for technical feasibility reasons. In such cases, the reading system has to consistently maintain the object's state — as reflected by the values of its properties and methods — across all copied instances. B.3 Methods B.3.1 hasFeature B.3.1.1 Description The hasFeature method indicates whether a reading system supports the specified feature. If no version is supplied with the named feature, the result indicates whether any version of the feature is supported. When a reading system supports a feature queried from the hasFeature method, it MUST return a boolean value indicating its support. If the feature is not recognized, the reading system MUST return undefined . The feature parameter specified in the hasFeature call MUST be identical to [ infra ] a supported feature for a reading system to return a true value. Note Refer to B.3.1.2 Features for a list of features that have to be supported. The version parameter allows querying of custom features that could change in incompatible ways over time. The return value indicates support only for the specified version of the feature. When the version parameter is set, its value MUST be identical to [ infra ] the version number supported by the reading system in order to return a true value. 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 1 : 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); B.3.1.2 Features The following table lists the set of features that reading systems that support the epubReadingSystem object MUST recognize. Name Description dom-manipulation Scripts can make structural changes to the document’s DOM (applies to spine-level scripting [ epub-34 ] only). layout-changes Scripts can modify attributes and CSS styles that affect content layout (applies to spine-level scripting [ epub-34 ] 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-34 ] (e.g., so a container-constrained script [ epub-34 ] 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. C. Index C.1 Terms defined by this specification content display area §1.2 epubReadingSystem §B. EpubReadingSystem interface §B.1 hasFeature method for EpubReadingSystem §B.3.1.1 Navigator.epubReadingSystem attribute for Navigator §B.2 obtain an expanded URL §11. C.2 Terms defined by reference [ BIDI ] defines the following: Rule P2 the higher-level protocols [ CSS-FONTS-4 ] defines the following: @font-face rules [ EPUB-33 ] defines the following: CSS Style Sheets — Prefixed properties Font obfuscation rendition:flow property [ EPUB-34 ] defines the following: active-class alternative text application/epub+zip application/oebps-package+xml Associating style information audio element audio core media type resources body element clipBegin clipEnd collections compact URLs container root URL container-constrained scripting content they read content URL core specification dc:creator element dc:identifier element dc:language element dc:title element declare an unknown value definition of epub:type deprecated authoring features dir disallowed duration EPUB container EPUB content documents EPUB content documents EPUB manifest EPUB navigation document EPUB navigation document EPUB navigation documents EPUB publications EPUB reading system EPUB spine epub:type attribute escapable structures Expressing in HTML Expressing the ICB in SVG Fallbacks for scripted content documents file name and file path restrictions fixed-layout document fixed-layout documents foreign content documents foreign resource fragment identifier global rendering property hreflang attributes identifier-type property image core media type resources item element itemref element landmarks nav element layout-pre-paginated override legacy features link element linked records linked resources links and headings manifest fallback chain manifest items manifest.xml file media overlay documents media overlays media-overlay attribute meta element metadata.xml files OCF abstract container OCF ZIP container outdated features package document package document package document metadata page-list nav element page-progression-direction attribute page-spread-left property page-spread-right property par element Parsing URLs in the META-INF directory playback-active-class preferred fallback order prefix primary (linear) itemref prohibiting CSS for setting language and direction properties attribute properties attribute properties attributes property attributes publication resources publication resources reference remote resources rendition:layout property rendition:page-spread-center property rendition:page-spread-left property rendition:page-spread-right property Reserved files reserved prefix Resource locations roll layouts root directory rootfile element scheme attribute scripted content documents scripting security and privacy section seq element signatures.xml file spine element spine-level scripting structural semantics SVG content documents SVG content documents synthetic spreads terminology defined in EPUB 3.4 text element text alternatives for visual content this version of EPUB toc nav element top-level content documents unique identifier values viewport viewport meta declarations visual rendering of XHTML content documents via CSS vocabulary association mechanisms XHTML content documents XHTML content documents xml:lang [ EPUB-TTS-10 ] defines the following: EPUB 3 Text-to-Speech Support [ HTML ] defines the following: attribute processing model conversion to JSON cookies DOMContentLoaded event is triggered form element hidden attribute (for html-global element) local storage Navigator interface navigator object object type origin suggested default rendering top-level browsing contexts web storage WorkerNavigator interface [ INFRA ] defines the following: is (for string ) null string strip and collapse ASCII whitespace [ ITS20 ] defines the following: processing ITS markup [ MATHML3 ] defines the following: annotation-xml elements Content MathML input-compliant processor Presentation MathML [ RDFA-CORE ] defines the following: attribute processing model [ URL ] defines the following: base origin path-relative-scheme-less-URL string scheme url parser URL-fragment string URL-scheme string valid URL string [ WEBIDL ] defines the following: boolean type DOMString interface [Exposed] extended attribute [LegacyUnforgeable] extended attribute [SameObject] extended attribute [ XML ] defines the following: external external identifiers internal xml:lang attribute D. Change log This section is non-normative. Note that this change log only identifies substantive changes since EPUB 3.3 — those that can affect the conformance of EPUB reading systems . For a list of all issues addressed, refer to the working group's issue tracker . Substantive changes since EPUB Reading Systems 3.3 14-Apr-2026: Clarified that the feature and version parameters of hasFeature are case sensitive for matching. See issue 2966 . 29-Jan-2026: Removed processing requirements for manifest fallbacks when used for content as this is now marked as an outdated feature. See issue 2900 . 21-Jan-2026: Clarified that the need to process fallbacks is dependent on the context in which a media type is used. See issue 2893 . 18-Dec-2025: Removed references to reflowable documents being allowed in spreads and clarified that spread placement properties only apply to pre-paginated documents. See pull request 2844 . 18-Dec-2025: Clarified support for outdated features. 18-Dec-2025: Removed processing for rendition:align-x-center as it now marked as a deprecated feature. See issue 2847 . 18-Dec-2025: Removed processing for the rendition:align-x-center , rendition:flow , rendition:orientation and rendition:spread properties and their overrides as these are now marked as outdated features. See issue 2841 . 18-Dec-2025: Added support for roll layouts in a new section under fixed layouts. See issue 2791 . 18-Dec-2025: Added a new "Layout" section to cover reflowable and fixed layouts. See pull request 2844 . 06-August-2025: Updated the section on handling CSS , in particular the relationships between the user agent's and the creators' style sheets. See discussion in issue 2762 and pull request 2771 . 05-June-2025: Consolidated all deprecated features under the unsupported features section. See the comments in pull request 2735 . 04-June-2025: Added section on optional ITS markup support. See issue 2732 . 08-Apr-2025: Added a requirement that reading systems ignore markup and styling intended to hide content in the EPUB navigation document when it is included in the spine. See issue 2687 . E. Acknowledgements This section is non-normative. The Publishing Maintenance Working Group would like to make a special acknowledgment to notable contributors, and friends, we have lost along the way. In particular, EPUB, not just EPUB 3, would not be what is today without the vision, knowledge, and preternatural good nature of Garth Conboy. Similarly, Ben Schroeter will always be remembered for his tireless work to help EPUB reach its potential as a universally accessible format. We are forever in their debt. The following members of the Publishing Maintenance Working Group contributed to the development of this specification: Laura Brady ( W3C Invited Expert) David Campbell (VitalSource Technologies) Gautier Chomel (EDRLab) Seth Delackner (Apple Inc.) Romain Deltour (DAISY Consortium) Marisa DeMeglio (DAISY Consortium) Laura Driussi (Google LLC) Brady Duga ( W3C Invited Expert) Elika Etemad (Apple Inc.) Jackie Fei (Taiwan Digital Publishing Forum) Symon Flaming (Rakuten Group, Inc.) Hadrien Gardeur (EDRLab) Matt Garrish (DAISY Consortium) Eloisa Guerrero (Rakuten Group, Inc.) David Hall (Apple Inc.) Eric Hellman (Free Ebook Foundation) Ivan Herman ( W3C , staff contact) Tetsu Hoshino (Kodansha, Publishers, Ltd.) Wen-Yen Huang (Taiwan Digital Publishing Forum) Yuki ICHIHARA (Kadokawa Corporation) Norikazu Ishizu (Kadokawa Corporation) Masato Ito (Kadokawa Corporation) Rick Johnson (VitalSource Technologies) Seong-Wook Joo (Google LLC) George Kerscher (DAISY Consortium) Masakazu Kitahara (Voyager Japan, Inc.) Toshiaki Koike (Voyager Japan, Inc.) Elizabeth Kraler (eBOUND Canada) Takashi Kuramoto (MEDIA DO Co., Ltd.) Ryo Kuroda (ACCESS CO., LTD.) Charles LaPierre (Benetech) Dan Lazin (Google LLC) Laurent Le Meur (EDRLab) Cristina Mussinelli (Fondazione LIA) Yoichiro Nagao (Kodansha, Publishers, Ltd.) Susan Neuhaus ( W3C Invited Expert, chair) Sophie Pang (Taiwan Digital Publishing Forum) Gregorio Pellegrino (Fondazione LIA) David Pham (The Washington Post) Stacy Ray (VitalSource Technologies) Wendy Reid ( W3C Invited Expert, chair) Milena Risi ( W3C Invited Expert) Dale Rogers ( W3C Invited Expert) John Roque (Apple Inc.) Leonard Rosenthol (Adobe) Madeleine Rothberg (National Center for Accessible Media at WGBH) Daihei Shiohama (MEDIA DO Co., Ltd.) Tzviya Siegman ( W3C ) Avneesh Singh (DAISY Consortium) Yutaka Suzuki (Kadokawa Corporation) Shinya Takami (Kadokawa Corporation, chair) Yukio Tomikura (Kodansha, Publishers, Ltd.) Daniel Weck (DAISY Consortium) Ivan Wong (Google LLC) Willy Yeh (Taiwan Digital Publishing Forum) Osamu Yoshiba (Kodansha, Publishers, Ltd.) Junichi Yoshii (Kodansha, Publishers, Ltd.) Naomi Yoshizawa ( W3C ) Permalink exported Referenced in: § 6.1.2.1 Pre-paginated layouts (2) § 7.3.1 Initial containing block dimensions (2) Permalink Referenced in: § 10. Processing structural semantics Permalink exported Referenced in: Not referenced in this document. Permalink exported Referenced in: § B.1 Interface definition Permalink exported IDL Referenced in: § B.1 Interface definition Permalink exported IDL Referenced in: § B.1 Interface definition Permalink Referenced in: § 5.1 Base direction Permalink Referenced in: § 5.1 Base direction Permalink Referenced in: § 7.4 Cascading Style Sheets (CSS) Permalink Referenced in: § 7.4 Cascading Style Sheets (CSS) Permalink Referenced in: § 4.3 Font obfuscation Permalink Referenced in: § 6.1.2.2 Roll layouts Permalink Referenced in: § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 15.2 Threat model Permalink Referenced in: § 15.2 Threat model Permalink Referenced in: § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 9.2.2.2 Rendering audio § 9.2.2.3 Rendering text-to-speech Permalink Referenced in: § 3.1 Core Media types Permalink Referenced in: § 9.2.2.1 Timing and synchronization Permalink Referenced in: § 9.2.2.2 Rendering audio Permalink Referenced in: § 9.2.2.2 Rendering audio Permalink Referenced in: § 5.6 Collections Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 4.1.1 URL of the root directory (2) (3) (4) (5) (6) (7) (8) Permalink Referenced in: § 7.5 Scripting § 7.5.3 Security considerations § B.2 Description § B.3.1.2 Features Permalink Referenced in: § 15.2 Threat model Permalink Referenced in: § 4.1.1 URL of the root directory Permalink Referenced in: § 1.1 Overview Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 3.8 Internationalization § 5.3 Metadata Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 10. Processing structural semantics (2) Permalink Referenced in: § A.2 Deprecated features Permalink Referenced in: § 3.8 Internationalization (2) § 5.1 Base direction Permalink Referenced in: § 4.1.1 URL of the root directory Permalink Referenced in: § 9.2.2.3 Rendering text-to-speech Permalink Referenced in: § 4. Open Container Format (OCF) processing § 15.2 Threat model Permalink Referenced in: § 1.2 Terminology § 1.4.1 Relationship to HTML § 3.3 Manifest fallbacks § 3.10 External links § 6.1.1 Reflowable layouts § 7.4 Cascading Style Sheets (CSS) § 7.5 Scripting § 7.5.3 Security considerations § 9.1 Text-to-speech § 9.2.1 Loading the media overlay § 9.2.2.3 Rendering text-to-speech § 9.2.2.4 Rendering EPUB content document elements § 9.2.3 Navigating within EPUB content documents § 10. Processing structural semantics § 11. Processing compact URLs § 14. Accessibility Permalink Referenced in: § 7. EPUB content document processing Permalink Referenced in: § 9.2 Media overlays processing § 9.2.1 Loading the media overlay Permalink Referenced in: § 9.2.3 Navigating within EPUB content documents (2) Permalink Referenced in: § 9.2.3 Navigating within EPUB content documents Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 1.1 Overview § 2.2 Error handling § 2.3 Error reporting § 4.1.1 URL of the root directory § 4.1.3 META-INF directory (2) § 5.2 Unique identifier § 5.3 Metadata § 5.4 Manifest § 5.5 Spine § 7.4.1 Author styling overrides § 7.5 Scripting § 7.5.3 Security considerations § 9.1 Text-to-speech § 9.2.2.3 Rendering text-to-speech § 9.2.3 Navigating within EPUB content documents § 12. Backward compatibility § 13. Forward compatibility § 14. Accessibility § 15.1 Overview § 15.2 Threat model § 15.3 Recommendations Permalink Referenced in: § 1.1 Overview § 1.2 Terminology § 1.4.1 Relationship to HTML § 1.4.2 Relationship to SVG § 2.1 Requirements § 3. Publication resource processing § 3.7 XML processing § 4. Open Container Format (OCF) processing § 5. Package document processing § 6.1.2.1 Pre-paginated layouts § 6.1.2.2 Roll layouts § 6.2 Custom properties § 7.1 XHTML content documents § 7.2 SVG content documents § 7.3 Fixed-layout documents § 7.5 Scripting § 8. Navigation document processing § 9.2 Media overlays processing § 10. Processing structural semantics § 11. Processing compact URLs (2) § 12. Backward compatibility § 13. Forward compatibility § 14. Accessibility § 15.1 Overview § A.2 Deprecated features § B. epubReadingSystem object § D. Change log Permalink Referenced in: § 6.1.2.2 Roll layouts § 8. Navigation document processing § 9.2.1 Loading the media overlay § 9.2.3 Navigating within EPUB content documents Permalink Referenced in: § 8. Navigation document processing § 9.2.4.1 Skippability § 9.2.4.2 Escapability § 10. Processing structural semantics § 11. Processing compact URLs Permalink Referenced in: § 9.2.4.2 Escapability Permalink Referenced in: § 7.3.1 Initial containing block dimensions Permalink Referenced in: § 7.3.1 Initial containing block dimensions Permalink Referenced in: § 7.5 Scripting Permalink Referenced in: § 4.1.2 File names Permalink Referenced in: § 2.3 Error reporting § 7.3.2 The viewport meta tag § 14. Accessibility Permalink Referenced in: § 7.5 Scripting Permalink Referenced in: § 3.3 Manifest fallbacks Permalink Referenced in: § 3.2 Foreign resources § 7.3 Fixed-layout documents § 7.3.1 Initial containing block dimensions Permalink Referenced in: § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 5.5.1 Spine overrides § 6.2 Custom properties Permalink Referenced in: § 3.8 Internationalization § 5.3 Metadata Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 3.1 Core Media types Permalink Referenced in: § 9.2 Media overlays processing § 9.2.1 Loading the media overlay § 11. Processing compact URLs Permalink Referenced in: § 2.1 Requirements § 5.5.1 Spine overrides § 6.1.2.1 Pre-paginated layouts § 6.1.2.1.1 Synthetic spread placement § 6.1.2.2 Roll layouts § 11. Processing compact URLs Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 5.5.1 Spine overrides § 6.1.2.2 Roll layouts Permalink Referenced in: § 5.7 Legacy features § A.1 Outdated features Permalink Referenced in: § 3.8 Internationalization § 11. Processing compact URLs Permalink Referenced in: § 3.10 External links Permalink Referenced in: § 5.3 Metadata § 5.4 Manifest Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 3.3 Manifest fallbacks (2) Permalink Referenced in: § 5.5 Spine Permalink Referenced in: § 4.1.3 META-INF directory Permalink Referenced in: § 3.8 Internationalization § 9.2.3 Navigating within EPUB content documents § 11. Processing compact URLs Permalink Referenced in: § 9.2 Media overlays processing Permalink Referenced in: § 9.2 Media overlays processing Permalink Referenced in: § 5.3 Metadata (2) § 6.1.1 Reflowable layouts § 11. Processing compact URLs (2) Permalink Referenced in: § 4.1.3 META-INF directory Permalink Referenced in: § 4.1.1 URL of the root directory § 4.2 OCF ZIP container Permalink Referenced in: § 4.2 OCF ZIP container Permalink Referenced in: § A.1 Outdated features Permalink Referenced in: § 2.1 Requirements § 3.8 Internationalization (2) § 3.10 External links § 4.1.3 META-INF directory § 9.2.1 Loading the media overlay § 11. Processing compact URLs § 12. Backward compatibility § 13. Forward compatibility Permalink Referenced in: § 5. Package document processing § 15.2 Threat model Permalink Referenced in: § 6.1.1 Reflowable layouts Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 5.5 Spine § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 9.2.2.1 Timing and synchronization Permalink Referenced in: § 4.1.1 URL of the root directory Permalink Referenced in: § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 3.3 Manifest fallbacks Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 5.5 Spine (2) § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 7. EPUB content document processing Permalink Referenced in: § 3.3 Manifest fallbacks Permalink Referenced in: § 5.4 Manifest Permalink Referenced in: § 5.5 Spine § 5.5.1 Spine overrides Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 3. Publication resource processing Permalink Referenced in: § 3.8 Internationalization (2) § 5.3 Metadata § 8. Navigation document processing Permalink Referenced in: § 11. Processing compact URLs (2) Permalink Referenced in: § 3.4 Resource locations § 15.2 Threat model (2) Permalink Referenced in: § 6.1.1 Reflowable layouts Permalink Referenced in: § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 4.1.3 META-INF directory Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 3.4 Resource locations Permalink Referenced in: § 6.1.2.2 Roll layouts Permalink Referenced in: § 4.1.1 URL of the root directory § 4.2 OCF ZIP container Permalink Referenced in: § 4.1.3 META-INF directory Permalink Referenced in: § 5.3 Metadata § 11. Processing compact URLs Permalink Referenced in: § 3.9 Network access § 7.5 Scripting § B.2 Description Permalink Referenced in: § 7.5 Scripting Permalink Referenced in: § 15.1 Overview Permalink Referenced in: § 9.2.2.1 Timing and synchronization Permalink Referenced in: § 4.1.3 META-INF directory Permalink Referenced in: § 5.5 Spine § 5.5.1 Spine overrides § 7.3 Fixed-layout documents § 7.3.1 Initial containing block dimensions § 9.1 Text-to-speech Permalink Referenced in: § 7.5 Scripting (2) § 7.5.3 Security considerations § B.3.1.2 Features (2) (3) Permalink Referenced in: § 10. Processing structural semantics Permalink Referenced in: § 3.8 Internationalization § 7.1.2.3.1 Embedded SVG and CSS § 7.3.1 Initial containing block dimensions § 9.2.1 Loading the media overlay Permalink Referenced in: § 7.2 SVG content documents Permalink Referenced in: § 1.2 Terminology § 6.1.2.1 Pre-paginated layouts § 6.1.2.1.1 Synthetic spread placement Permalink Referenced in: § 1.2 Terminology Permalink Referenced in: § 9.2.2.3 Rendering text-to-speech § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 14. Accessibility Permalink Referenced in: § 5.7 Legacy features § 12. Backward compatibility § 13. Forward compatibility Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 3.5 Data URLs § B.3.1.2 Features Permalink Referenced in: § 5.2 Unique identifier Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 1.2 Terminology § 2.1 Requirements § 3.1 Core Media types § 6.1.2.1 Pre-paginated layouts § 7.1.2.2 MathML § 7.2 SVG content documents § 7.4 Cascading Style Sheets (CSS) § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 7.3.2 The viewport meta tag Permalink Referenced in: § 7.4 Cascading Style Sheets (CSS) Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 3.8 Internationalization § 5.5 Spine § 6.1.2.1.1 Synthetic spread placement § 7.1.2.2 MathML § 7.1.2.3 Embedded SVG § 7.1.2.3.1 Embedded SVG and CSS § 7.3.1 Initial containing block dimensions § 7.3.2 The viewport meta tag § 9.2.1 Loading the media overlay § 9.2.3 Navigating within EPUB content documents Permalink Referenced in: § 7.1 XHTML content documents Permalink Referenced in: § 3.8 Internationalization Permalink Referenced in: § 9.2.2.3 Rendering text-to-speech Permalink Referenced in: § 7.1.2.1 Microdata Permalink Referenced in: § 7.1.2.1 Microdata Permalink Referenced in: § 7.5.1 Local storage § 7.5.3 Security considerations Permalink Referenced in: § B.2 Description Permalink Referenced in: § 7.1.2.4 Form submission Permalink Referenced in: § 8. Navigation document processing Permalink Referenced in: § 7.5.1 Local storage Permalink Referenced in: § B.1 Interface definition (2) Permalink Referenced in: § 7.5 Scripting Permalink Referenced in: § 7.1.2.3.1 Embedded SVG and CSS Permalink Referenced in: § 4.1.1 URL of the root directory § 7.5 Scripting (2) § 7.5.3 Security considerations Permalink Referenced in: § 7.4 Cascading Style Sheets (CSS) Permalink Referenced in: § 3.5 Data URLs Permalink Referenced in: § 7.5.1 Local storage § 7.5.3 Security considerations Permalink Referenced in: § B.1 Interface definition Permalink Referenced in: § B.3.1.1 Description (2) Permalink Referenced in: § 11. Processing compact URLs (2) (3) (4) (5) (6) Permalink Referenced in: § 11. Processing compact URLs (2) (3) Permalink Referenced in: § 5.3 Metadata Permalink Referenced in: § 7.1.1.2 Internationalization Tag Set (ITS) Permalink Referenced in: § 7.1.2.2 MathML Permalink Referenced in: § 7.1.2.2 MathML Permalink Referenced in: § 7.1.2.2 MathML Permalink Referenced in: § 7.1.2.2 MathML Permalink Referenced in: § 7.1.1.1 RDFa Permalink Referenced in: § 4.1.1 URL of the root directory (2) (3) Permalink Referenced in: § 4.1.1 URL of the root directory Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 3.10 External links Permalink Referenced in: § 4.1.1 URL of the root directory (2) § 11. Processing compact URLs Permalink Referenced in: § 9.2.2.4 Rendering EPUB content document elements Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § 11. Processing compact URLs Permalink Referenced in: § B.1 Interface definition Permalink Referenced in: § B.1 Interface definition (2) Permalink Referenced in: § B.1 Interface definition Permalink Referenced in: § B.1 Interface definition Permalink Referenced in: § B.1 Interface definition Permalink Referenced in: § 15.3 Recommendations Permalink Referenced in: § 15.3 Recommendations Permalink Referenced in: § 15.3 Recommendations Permalink Referenced in: § 3.8 Internationalization F. References F.1 Normative references [bidi] Unicode Bidirectional Algorithm . Manish Goregaokar मनीष गोरेगांवकर; Robin Leroy. Unicode Consortium. 13 August 2025. Unicode Standard Annex #9. URL: https://www.unicode.org/reports/tr9/tr9-51.html [css-fonts-4] CSS Fonts Module Level 4 . Chris Lilley. W3C. 22 April 2026. W3C Working Draft. URL: https://www.w3.org/TR/css-fonts-4/ [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/CSS2/ [csssnapshot] CSS Snapshot . URL: https://www.w3.org/TR/CSS/ [dcterms] DCMI Metadata Terms . DCMI Usage Board. DCMI. 20 January 2020. DCMI Recommendation. URL: https://www.dublincore.org/specifications/dublin-core/dcmi-terms/ [dom] DOM Standard . Anne van Kesteren. WHATWG. Living Standard. URL: https://dom.spec.whatwg.org/ [epub-33] EPUB 3.3 . Ivan Herman; Matt Garrish; Dave Cramer. W3C. 13 January 2026. W3C Recommendation. URL: https://www.w3.org/TR/epub-33/ [epub-34] EPUB 3.4 . Ivan Herman; Matt Garrish. W3C. 21 July 2026. W3C Candidate Recommendation. URL: https://www.w3.org/TR/epub-34/ [epubcontentdocs-301] EPUB Content Documents 3.0.1 . Markus Gylling; William McCoy; Elika J. Etimad; Matt Garrish. IDPF. 26 June 2014. URL: https://idpf.org/epub/301/spec/epub-contentdocs-20140626.html [epubcontentdocs-32] EPUB Content Documents 3.2 . Dave Cramer; Matt Garrish. W3C. 08 May 2019. URL: https://www.w3.org/publishing/epub32/epub-contentdocs.html [epubmediaoverlays-32] EPUB Media Overlays 3.2 . Marisa DeMeglio; Daniel Weck. EPUB 3 Community Group. 08 May 2019. URL: https://www.w3.org/publishing/epub32/epub-mediaoverlays.html [html] HTML Standard . Anne van Kesteren; Domenic Denicola; Dominic Farolino; 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/ [its20] Internationalization Tag Set (ITS) Version 2.0 . David Filip; Shaun McCance; David Lewis; Christian Lieske; Arle Lommel; Jirka Kosek; Felix Sasaki; Yves Savourel. W3C. 29 October 2013. W3C Recommendation. URL: https://www.w3.org/TR/its20/ [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/ [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/ [rfc1951] DEFLATE Compressed Data Format Specification version 1.3 . P. Deutsch. IETF. May 1996. Informational. URL: https://www.rfc-editor.org/info/rfc1951/ [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/info/rfc2119/ [rfc2397] The "data" URL scheme . L. Masinter. IETF. August 1998. Proposed Standard. URL: https://www.rfc-editor.org/info/rfc2397/ [rfc8089] The "file" URI Scheme . M. Kerwin. IETF. February 2017. Proposed Standard. URL: https://www.rfc-editor.org/info/rfc8089/ [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/info/rfc8174/ [rfc9110] HTTP Semantics . R. Fielding, Ed.; M. Nottingham, Ed.; J. Reschke, Ed. IETF. June 2022. Internet Standard. URL: https://httpwg.org/specs/rfc9110.html [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/policies/process/ [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. W3C. 8 August 2024. 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/ [xml-names] Namespaces in XML 1.0 (Third Edition) . Tim Bray; Dave Hollander; Andrew Layman; Richard Tobin; Henry Thompson et al. W3C. 8 December 2009. W3C Recommendation. URL: https://www.w3.org/TR/xml-names/ [xmlenc-decrypt] Decryption Transform for XML Signature . Merlin Hughes; Takeshi Imamura; Hiroshi Maruyama. W3C. 10 December 2002. W3C Recommendation. URL: https://www.w3.org/TR/xmlenc-decrypt/ [zip] .ZIP File Format Specification . 15 July 2020. Final. URL: https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT F.2 Informative references [epub-tts-10] EPUB 3 Text-to-Speech Enhancements 1.0 . Matt Garrish. W3C. 28 August 2025. W3C Working Group Note. URL: https://www.w3.org/TR/epub-tts-10/ [fips-180-4] FIPS PUB 180-4: Secure Hash Standard (SHS) . U.S. Department of Commerce/National Institute of Standards and Technology. August 2015. National Standard. URL: https://nvlpubs.nist.gov/nistpubs/FIPS/NIST.FIPS.180-4.pdf [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 [indexeddb] Indexed Database API . Nikunj Mehta; Jonas Sicking; Eliot Graff; Andrei Popescu; Jeremy Orlow; Joshua Bell. W3C. 8 January 2015. W3C Recommendation. URL: https://www.w3.org/TR/IndexedDB/ [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/info/rfc6386/ [ruby-tts-req] Text-to-Speech Rendering of Electronic Documents Containing Ruby: User Requirements . Makoto Murata. W3C. 25 April 2026. DNOTE. URL: https://www.w3.org/TR/ruby-tts-req/ [uaag20] User Agent Accessibility Guidelines (UAAG) 2.0 . James Allan; Greg Lowney; Kimberly Patch; Jeanne F Spellman. W3C. 15 December 2015. W3C Working Group Note. URL: https://www.w3.org/TR/UAAG20/ [wai-aria-11] Accessible Rich Internet Applications (WAI-ARIA) 1.1 . Joanmarie Diggs; Shane McCarron; Michael Cooper; Richard Schwerdtfeger; James Craig. W3C. 14 December 2017. W3C Recommendation. URL: https://www.w3.org/TR/wai-aria-1.1/ ↑

Related documents

• latn gap#649807
• grek gap#649808
• mong gap#649809
• nkoo gap#649810
• osge osa gap#649811