INTERNATIONAL ISO/IEC
STANDARD 23736-5
First edition
2020-02
Information technology — Digital
publishing — EPUB 3.0.1 —
Part 5:
Media overlays
Technologies de l'information — Publications numériques — EPUB
3.0.1 —
Partie 5: Superposition de médias
Reference number
©
ISO/IEC 2020
© ISO/IEC 2020
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Published in Switzerland
ii © ISO/IEC 2020 – All rights reserved

Foreword
ISO (the International Organization for Standardization) and IEC (the International Electrotechnical
Commission) form the specialized system for worldwide standardization. National bodies that are
members of ISO or IEC participate in the development of International Standards through technical
committees established by the respective organization to deal with particular fields of technical activity.
ISO and IEC technical committees collaborate in fields of mutual interest. Other international
organizations, governmental and non‐governmental, in liaison with ISO and IEC, also take part in the
work.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of document should be noted (see www.iso.org/directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. ISO and IEC shall not be held responsible for identifying any or all such patent rights.
Details of any patent rights identified during the development of the document will be in the
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list of patent declarations received (see http://patents.iec.ch).
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT),
see www.iso.org/iso/foreword.html.
This document was prepared by the World Wide Web Consortium (W3C) (as EPUB Media Overlays
3.0.1) and drafted in accordance with its editorial rules. It was adopted, under the JTC 1 PAS
procedure, by Joint Technical Committee ISO/IEC JTC 1, Information technology.
A list of all parts in the ISO/IEC 23736 series can be found on the ISO websitte.e
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
© ISO/IEC 2020 – All rights reserved iii

EPUB Media Overlays 3.0.1
Recommended Specification 26 June 2014
THIS VERSION
http://www.idpf.org/epub/301/spec/epub-mediaoverlays-20140626.html
LATEST VERSION
http://www.idpf.org/epub3/latest/mediaoverlays
PREVIOUS VERSION
http://www.idpf.org/epub/301/spec/epub-mediaoverlays-20140228.html
A diff of changes from the previous version is also available.
Please refer to the errata for this document, which may include some normative corrections.
All rights reserved. This work is protected under Title 17 of the United States Code. Reproduction and
dissemination of this work with changes is prohibited except with the written permission of the International
Digital Publishing Forum (IDPF).
EPUB is a registered trademark of the International Digital Publishing Forum.
Editors
Marisa DeMeglio, DAISY Consortium
Daniel Weck, DAISY Consortium
TABLE OF CONTENTS
1. Overview
1.1. Purpose and Scope
1.2. Relationship to Other Specifications
1.3. Terminology
1.4. Typographic Conventions
1.5. Conformance Statements
1.6. Namespace prefix mappings
2. Media Overlay Document Definition
2.1. Introduction
2.2. Content Conformance
2.3. Reading System Conformance
2.4. Media Overlay Document Definition
2.4.1. The smil Element
2.4.2. The head Element
2.4.3. The metadata Element
2.4.4. The body Element
2.4.5. The seq Element
2.4.6. The par Element
2.4.7. The text Element
2.4.8. The audio Element
© ISO/IEC 2020 – All rights reserved 1

3. Creating Media Overlays
3.1. Overview
3.2. Relationship to the EPUB Content Document
3.2.1. Structure
3.2.2. Granularity
3.2.3. Embedded Audio and Video
3.2.4. Text-to-Speech
3.3. Semantic Inflection
3.4. Associating Style Information
3.5. Packaging
3.5.1. Including Media Overlays
3.5.2. Media Overlays Metadata Vocabulary
4. Playback Behaviors
4.1. Loading the Media Overlay
4.2. Basic Playback
4.2.1. Timing and Synchronization
4.2.2. Rendering Audio
4.2.3. Rendering EPUB Content Document Elements
4.3. Interacting with the EPUB Content Document
4.3.1. Navigation
4.3.2. Embedded Audio and Video
4.3.3. Text-to-Speech
4.4. Skippability and Escapability
4.4.1. Skippability
4.4.2. Escapability
A. Media Overlays Schema
B. Examples of Clock Values
C. Acknowledgements and Contributors
References
› 1 Overview
› 1.1 Purpose and Scope
This section is informative
This specification, EPUB Media Overlays 3.0.1, defines a usage of [SMIL] (Synchronized Multimedia
Integration Language), the Package Document, the EPUB® Style Sheet, and the EPUB Content
Document for representation of audio synchronized with the EPUB Content Document.
This specification is one of a family of related specifications that compose EPUB 3, the third major
revision of an interchange and delivery format for digital publications based on XML and Web
Standards. It is meant to be read and understood in concert with the other specifications that make up
EPUB 3:
The EPUB 3 Overview [EPUB3Overview], which provides an informative overview of EPUB and
a roadmap to the rest of the EPUB 3 documents. The Overview should be read first.
EPUB Publications 3.0.1 [Publications301], which defines the semantics and overarching
conformance requirements for each Rendition of an EPUB Publication.
EPUB Content Documents 3.0.1 [ContentDocs301], which defines profiles of XHTML, SVG and
CSS for use in the context of EPUB Publications.
2 © ISO/IEC 2020 – All rights reserved

EPUB Open Container Format (OCF) 3.0.1 [OCF301], which defines a file format and
processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB
Container.
› 1.2 Relationship to Other Specifications
This section is informative
This specification relies on a subset of [SMIL], from which the EPUB Media Overlays elements and
attributes defined in Media Overlay Document Definition are derived.
› 1.3 Terminology
EPUB Publication
A collection of one or more Renditions conforming to this specification and its sibling
specifications , packaged in an EPUB Container.
An EPUB Publication typically represents a single intellectual or artistic work, but this
specification and its sibling specifications do not circumscribe the nature of the content.
Rendition
A logical document entity consisting of a set of interrelated resources representing one
rendering of an EPUB Publication.
Publication Resource
A resource that contains content or instructions that contribute to the logic and rendering of
at least one Rendition of an EPUB Publication. In the absence of this resource, the EPUB
Publication might not render as intended by the Author. Examples of Publication Resources
include a Rendition's Package Document, EPUB Content Document, EPUB Style Sheets,
audio, video, images, embedded fonts and scripts.
With the exception of the Package Document itself, the Publication Resources required to
render a Rendition are listed in that Rendition's manifest [Publications301] and bundled in
the EPUB Container file (unless specified otherwise in Publication Resource Locations
[Publications301] ).
Examples of resources that are not Publication Resources include those identified by the
Package Document link [Publications301] element and those identified in outbound
hyperlinks that resolve outside the EPUB Container (e.g., referenced from an [HTML5] a
element href attribute).
EPUB Content Document
A Publication Resource that conforms to one of the EPUB Content Document definitions
(XHTML or SVG).
An EPUB Content Document is a Core Media Type, and may therefore be included in the
EPUB Publication without the provision of fallbacks [Publications301] .
XHTML Content Document
© ISO/IEC 2020 – All rights reserved 3

An EPUB Content Document conforming to the profile of [HTML5] defined in XHTML
Content Documents [ContentDocs301] .
XHTML Content Documents use the XHTML syntax of [HTML5].
SVG Content Document
An EPUB Content Document conforming to the constraints expressed in SVG Content
Documents [ContentDocs301] .
EPUB Navigation Document
A specialization of the XHTML Content Document, containing human- and machine-
readable global navigation information, conforming to the constraints expressed in EPUB
Navigation Documents [ContentDocs301] .
Core Media Type
A set of Publication Resource types for which no fallback is required. Refer to Publication
Resources [Publications301] for more information.
Package Document
A Publication Resource carrying bibliographical and structural metadata about a given
Rendition of an EPUB Publication, as defined in Package Documents [Publications301] .
Manifest
A list of all Publication Resources that constitute the given Rendition of a EPUB
Publication.
Refer to manifest [Publications301] for more information.
Spine
An ordered list of Publication Resources, typically EPUB Content Documents, representing
the default reading order of the given Rendition of an EPUB Publication.
Refer to spine [Publications301] for more information.
Media Overlay Document
An XML document that associates the XHTML Content Document with pre-recorded audio
narration in order to provide a synchronized playback experience, as defined in this
specification.
Text-to-Speech (TTS)
The rendering of the textual content of an EPUB Publication as artificial human speech
using a synthesized voice.
EPUB Style Sheet (or Style Sheet)
A CSS Style Sheet conforming to the CSS profile defined in EPUB Style Sheets
[ContentDocs301] .
Viewport
The region of an EPUB Reading System in which the content of an EPUB Publication is
rendered visually to a User.
CSS Viewport
4 © ISO/IEC 2020 – All rights reserved

A Viewport capable of displaying CSS-styled content.
EPUB Container (or Container)
The ZIP-based packaging and distribution format for EPUB Publications defined in
[OCF301].
Author
The person(s) or organization responsible for the creation of an EPUB Publication, which is
not necessarily the creator of the content and resources it contains.
User
An individual that consumes an EPUB Publication using an EPUB Reading System.
EPUB Reading System (or Reading System)
A system that processes EPUB Publications for presentation to a User in a manner
conformant with this specification and its sibling specifications .
› 1.4 Typographic Conventions
The following typographic conventions are used in this specification:
markup
All markup (elements, attributes, properties), code (JavaScript, pseudo-code), machine
processable values (string, characters, media types) and file names are in red-orange
monospace font.
markup
Links to markup and code definitions are underlined and in red-orange monospace font. Only
the first instance in each section is linked.
http://www.idpf.org/
URIs are in navy blue monospace font.
hyperlink
Hyperlinks are underlined and in blue.
[reference]
Normative and informative references are enclosed in square brackets.
Term
Terms defined in the Terminology are in capital case.
Term
Links to term definitions have a dotted blue underline. Only the first instance in each section is
linked.
Normative element, attribute and property definitions are in blue boxes.
© ISO/IEC 2020 – All rights reserved 5

Informative markup examples are in white boxes.
NOTE
Informative notes are in yellow boxes with a "Note" header.
CAUTION
Informative cautionary note are in red boxes with a "Caution" header.
› 1.5 Conformance Statements
The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD, SHOULD NOT, RECOMMENDED, MAY,
and OPTIONAL in this document are to be interpreted as described in [RFC2119].
All sections of this specification are normative except where identified by the informative status label
"This section is informative". The application of informative status to sections and appendices applies
to all child content and subsections they may contain.
All examples in this specification are informative.
› 1.6 Namespace prefix mappings
For convenience, the following namespace prefix mappings [XMLNS] are used throughout this
specification:
prefix namespace URI
epub http://www.idpf.org/2007/ops
› 2 Media Overlay Document Definition
› 2.1 Introduction
This section is informative
Books featuring synchronized audio narration are found in mainstream e-books, educational tools and
e-books formatted for persons with print disabilities. In EPUB 3, these types of books are created by
using Media Overlay Documents to describe the timing for the pre-recorded audio narration and how
6 © ISO/IEC 2020 – All rights reserved

it relates to the EPUB Content Document markup. The file format for Media Overlays is defined as a
subset of SMIL, a W3C recommendation for representing synchronized multimedia information in
XML.
The Media Overlays feature is designed to be transparent to EPUB Reading Systems that do not
support the feature. The inclusion of Media Overlays in a Rendition of an EPUB Publication has no
impact on the ability of Media Overlay-unaware Reading Systems to render that Rendition as though
the Media Overlays are not present.
Although future versions of this specification may incorporate support for video media (e.g.,
synchronized text/sign-language books), this version supports only synchronizing audio media with
the EPUB Content Document.
› 2.2 Content Conformance
A Media Overlay Document MUST meet all of the following criteria:
Document Properties
›
It MUST meet the conformance constraints for XML documents defined in XML Conformance
[Publications301] .
›
It MUST be valid to the Media Overlays schema as defined in Appendix A, Media Overlays
Schema and conform to all content conformance constraints expressed in Media Overlay
Document Definition.
›
It MUST be authored to reflect the structure of the EPUB Content Document with which it is
associated, as stated in Structure .
›
Authors SHOULD avoid using scripts to control audio and video embedded in the EPUB Content
Document, as stated in Embedded Audio and Video.
›
It SHOULD use semantic markup where appropriate, as described in Semantic Inflection.
›
It MUST be packaged with the EPUB Publication as shown in Packaging.
File Properties
›
The Media Overlay Document filename SHOULD use the file extension .smil.
› 2.3 Reading System Conformance
EPUB Reading System support for Media Overlays is OPTIONAL. A Reading System that supports
Media Overlays MUST meet the following criteria:
›
It MUST process the Media Overlay Document in conformance with all Reading System
conformance constraints expressed in Media Overlay Document Definition.
›
It MUST support XHTML Content Documents, and it MAY support SVG Content Documents.
›
It MUST render Media Overlay elements as described in Basic Playback.
© ISO/IEC 2020 – All rights reserved 7

›
It MUST allow User navigation while a Media Overlay is being played, as discussed in
Navigation.
›
It MUST adhere to rules regarding referenced audio and video embedded in the EPUB Content
Document, as stated in Embedded Audio and Video.
›
Text-to-Speech (TTS)-capable Reading Systems SHOULD conform to Reading System Text-to-
Speech Conformance Requirements [Publications301] .
›
It SHOULD offer the skippability and escapability features described in Skippability and
Escapability.
A Reading System that does not support Media Overlays MUST meet the following criteria:
›
It MUST ignore both the media-overlay attribute on manifest item elements and the manifest
item elements where the media-type attribute value equals application/smil+xml.
› 2.4 Media Overlay Document Definition
All elements [XML] defined in this section are in the http://www.w3.org/ns/SMIL namespace [XMLNS]
unless otherwise specified.
› 2.4.1 The smil Element
The smil element MUST be the root element of all Media Overlay Documents.
Element Name
smil
Usage
The smil element is the root element of the Media Overlay Document.
Attributes
version [required]
Specifies the version number of the [SMIL] specification to which the Media Overlay
adheres.
This attribute MUST have the value "3.0" to indicate compliance with this version of the
specification.
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
epub:prefix [optional]
Declares additional metadata vocabulary prefixes.
Refer to Semantic Inflection for more information.
8 © ISO/IEC 2020 – All rights reserved

Content Model
In this order: head [optional], body [required]
› 2.4.2 The head Element
The head element is the container for metadata in the Media Overlay Document, and consists of zero
or one child metadata element.
Element Name
head
Usage
The head element is the optional first child of the smil element.
Attributes
None.
Content Model
metadata [0 or 1].
As this specification defines no metadata properties that must occur in the Media Overlay Document,
the head element is optional.
› 2.4.3 The metadata Element
The metadata element represents metadata for the Media Overlay Document. The metadata element
is an extension point that allows the inclusion of metadata from any metainformation structuring
language.
Element Name
metadata
Usage
As a child of the head element.
Attributes
None.
Content Model
[0 or more] elements from any namespace.
© ISO/IEC 2020 – All rights reserved 9

This specification defines no metadata properties that MUST occur in the Media Overlay Document;
the metadata element is provided for custom metadata requirements.
› 2.4.4 The body Element
The body element is the starting point for the presentation contained in the Media Overlay Document.
It contains the main sequence of par and seq elements.
Element Name
body
Usage
The body element is the required second child of the smil element.
Attributes
epub:type [optional]
An expression of the structural semantics of the corresponding element in the EPUB
Content Document.
The value is a whitespace separated list of property [Publications301] types. Refer to
Semantic Inflection for more information.
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
epub:textref [optional]
The relative IRI reference [RFC3987] of the corresponding EPUB Content Document,
including a fragment identifier that references the specific element as per the
[XPTRSH].
Content Model
In any order: seq [0 or more] or par [0 or more]
At least one par or seq is required.
› 2.4.5 The seq Element
The seq element contains media objects which are to be rendered sequentially.
Element Name
seq
Usage
10 © ISO/IEC 2020 – All rights reserved

One or more seq elements MAY occur as children of the body element and of the seq
element.
Attributes
epub:type [optional]
An expression of the structural semantics of the corresponding element in the EPUB
Content Document.
The value is a whitespace separated list of property [Publications301] types. Refer to
Semantic Inflection for more information.
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
epub:textref [required]
The relative IRI reference [RFC3987] of the corresponding EPUB Content Document,
including a fragment identifier that references the specific element as per the
[XPTRSH].
Content Model
In any order: seq [0 or more] or par [0 or more].
At least one par or seq is required.
› 2.4.6 The par Element
The par element contains media objects which are to be rendered in parallel.
Element Name
par
Usage
One or more par elements MAY occur as children of the body and seq elements.
Attributes
epub:type [optional]
An expression of the structural semantics of the corresponding element in the EPUB
Content Document.
The value is a whitespace separated list of property [Publications301] types. Refer to
Semantic Inflection for more information.
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
© ISO/IEC 2020 – All rights reserved 11

Content Model
In any order: text [required] and audio [optional]
The audio element is optional only if its sibling text element refers to audio or video media
(see Embedded Audio and Video), or to textual content intended for rendering via Text-to-
Speech (TTS).
› 2.4.7 The text Element
The text element references an element in the EPUB Content Document. A text element typically
refers to a textual element, but can also refer to other EPUB Content Document media elements (see
Embedded Audio and Video).
Element Name
text
Usage
As a required child of the par element.
Attributes
src [required]
The relative IRI reference [RFC3987] of the corresponding EPUB Content Document,
including a fragment identifier that references the specific element as per the
[XPTRSH].
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
Content Model
Empty.
› 2.4.8 The audio Element
The audio element represents a clip of audio media.
Element Name
audio
Usage
A required child of the par element unless its sibling text element refers to audio or video
media, in which case it is optional (see Embedded Audio and Video).
12 © ISO/IEC 2020 – All rights reserved

Attributes
id [optional]
The ID [XML] of this element, which MUST be unique within the document scope.
src [required]
The relative or absolute IRI reference [RFC3987] of an audio file. The audio file MUST
be one of the audio formats listed in the Core Media Types [Publications301] table.
clipBegin [optional]
A clock value that specifies the offset into the physical media corresponding to the
start point of an audio clip.
Clock values are a subset of SMIL clock values, defined in [SMIL]. See Appendix B,
Examples of Clock Values .
clipEnd [optional]
A clock value that specifies the offset into the physical media corresponding to the
end point of an audio clip.
Clock values are a subset of SMIL clock values, defined in [SMIL]. See Appendix B,
Examples of Clock Values .
The chronological offset of the terminating position MUST be after the starting offset
specified in the clipBegin attribute.
Content Model
Empty.
› 3 Creating Media Overlays
› 3.1 Overview
This section is informative
A pre-recorded narration of a publication can be represented as a series of audio clips, each
corresponding to part of the EPUB Content Document. A single audio clip, for example, typically
represents a single phrase or paragraph, but infers no order relative to the other clips or to the text of
a document. Media Overlays solve this problem of synchronization by tying the structured audio
narration to its corresponding text (or other media) in the EPUB Content Document using SMIL
markup. Media Overlays are, in fact, a simplified subset of SMIL 3.0 that allow the playback sequence
of these clips to be defined.
The SMIL elements primarily used for structuring Media Overlays are body (used for the main
sequence), seq (sequence) and par (parallel). (Refer to Media Overlay Document Definition for more
information on these and other SMIL elements.)
© ISO/IEC 2020 – All rights reserved 13

The par element is the basic building block of an Overlay and corresponds to a phrase in the EPUB
Content Document. The element provides two key pieces of information for synchronizing content: 1)
the audio clip containing the narration for the phrase; and 2) a pointer to the associated EPUB
Content Document fragment. The par element uses two media element children to represent this
information: an audio element and a text element. Since par elements render their children in
parallel, the audio clip and EPUB Content Document fragment are played at the same time, resulting
in a synchronized presentation.
The text element src attribute references the associated phrase, sentence, or other segment of the
EPUB Content Document by its IRI reference. The audio element src attribute similarly references
the location of the corresponding audio clip, and adds the optional clipBegin and clipEnd attributes
to indicate a specific offset within the clip.
The following example shows the Media Overlays markup for a single phrase or sentence.



par elements are placed together sequentially to form a series of phrases or sentences. Not every
element of the EPUB Content Document will have a corresponding par element in the Media Overlay,
only those relevant to the audio narration.
The following example shows a basic Media Overlay Document containing a sequence of phrases. The
body element acts as the main sequence for the whole document.
version="3.0">












par elements can also be added to seq elements to define more complex structures such as parts and
chapters (see Structure ).
› 3.2 Relationship to the EPUB Content Document
14 © ISO/IEC 2020 – All rights reserved

In this section, the EPUB Content Document is assumed to be an XHTML Content
NOTE
Document. While Media Overlays can be used with SVG Content Documents, playback
behavior might not be consistent and therefore interoperability is not guaranteed.
› 3.2.1 Structure
The ordering of the Media Overlay elements MUST match the default reading order of the EPUB
Content Document. The par element represents phrases, and the seq element (sequence)
represents nested EPUB Content Document containers such as sections, asides, headers, and
footnotes. seq children MUST be other seq or par elements. Each seq element MUST contain an
epub:textref attribute which references the corresponding EPUB Content Document element by IRI
reference.
The following example shows a Media Overlay Document with nested seq elements, representing a
chapter with both a section header and a sidebar, which itself has a nested figure.
xmlns:epub="http://www.idpf.org/2007/ops"
version="3.0">


epub:type="chapter">












epub:type="sidebar">




© ISO/IEC 2020 – All rights reserved 15



























NOTE
The reason for grouping structures like sidebars, section headers, figures, tables, and
footnotes in a seq element is so that their start and end positions can be identified during
playback. Reading Systems can then offer playback options tailored to the layout of the given
Rendition, such as jumping past a long sidebar, turning off rendering of page break
announcements (see Skippability and Escapability), or customizing the reading mode to suit
structures such as tables.
The following example shows the EPUB Content Document that corresponds to the previous Media
Overlay example.
xmlns:epub="http://www.idpf.org/2007/ops"
16 © ISO/IEC 2020 – All rights reserved

xml:lang="en"
lang="en">

Media Overlays Example of EPUB Content Document