This specification defines a usage of [[SMIL3]] (Synchronized Multimedia Integration Language), the Package Document, CSS Style Sheets, and EPUB® Content Documents for representation of audio synchronized with the EPUB Content Document.
The text and audio synchronization enabled by Media Overlays provides enhanced accessibility for any user who has difficulty following the text of a traditional book. Media Overlays also provide a continuous listening experience for readers who are unable to read the text for any reason, something that traditional audio embedding techniques cannot offer. They are even useful for purposes not traditionally considered accessibility concerns (e.g., for language learning or reading of commercial audio books).
This specification is one of a family of specifications that compose EPUB 3 [[EPUB32]], 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.
Refer to [[EPUB32Changes]] for more information on the differences between this specification and its predecessor.
This specification relies on a subset of [[SMIL3]], from which the EPUB Media Overlays elements and attributes defined in Media Overlay Document Definition are derived.
Terms with meanings specific to EPUB 3 are capitalized in this document (e.g., "Author", "Reading System"). A complete list of these terms and definitions is provided in [[!EPUB32]].
Only the first instance of a term in a section is linked to its definition.
For convenience, the following namespace prefixes [[!XML-NAMES]] are used in this specification without explicitly being declared. To use any of these prefixes, a declaration is REQUIRED.
| prefix | URI |
|---|---|
epub
|
http://www.idpf.org/2007/ops
|
Synchronized audio narration is found in mainstream ebooks, educational tools and ebooks formatted for persons with print disabilities. In EPUB 3, these types of books are created using Media Overlay Documents to describe the timing for the pre-recorded audio narration and how it relates to the EPUB Content Document markup. The file format for Media Overlays is defined as a subset of [[SMIL3]], 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 an EPUB Publication has no impact on the ability of Media Overlay-unaware Reading Systems to render the EPUB Publication as though the Media Overlays are not present.
Although future versions of this specification might incorporate support for video media (e.g., synchronized text/sign-language books), this version supports only synchronizing audio media with the EPUB Content Document.
A Media Overlay Document MUST meet all of the following criteria:
It MUST meet the conformance constraints for XML documents defined in XML Conformance [[!EPUB32]].
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.
It MAY refer to more than one EPUB Content Document, but an EPUB Content Document MUST NOT be referenced by more than one Media Overlay Document.
It MUST adhere to the requirements for Embedded Media.
It SHOULD use semantic markup where appropriate, as described in Semantic Inflection.
It MUST be packaged with the EPUB Publication as shown in Packaging.
The Media Overlay Document filename SHOULD use the file
extension .smil.
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.
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-capable Reading Systems SHOULD conform to Reading System Text-to-Speech Conformance Requirements [[!EPUB32]].
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.
All elements [[!XML]] defined in this section are in the https://www.w3.org/ns/SMIL
namespace [[!XML-NAMES]] unless otherwise specified.
smil ElementThe smil element is the root element of all Media Overlay Documents.
smil
The smil element is the root element of the Media Overlay Document.
version
[required]
Specifies the version number of the [[!SMIL3]] specification to which the Media Overlay adheres.
This attribute MUST have the value "3.0".
id
[optional]
The ID [[!XML]] of the element, which MUST be unique within the document scope.
epub:prefix
[optional]
Declares additional metadata vocabulary prefixes.
Refer to Semantic Inflection for more information.
In this order:
head ElementThe head element is the container for metadata in the Media Overlay Document.
head
The head element is the OPTIONAL first child of the smil element.
None
metadata
[0 or 1]
As this specification does not define any metadata properties that have to occur in the Media
Overlay Document, the head element is OPTIONAL.
metadata ElementThe 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.
metadata
As a child of the head element.
None
[0 or more] elements from any namespace
This specification defines no metadata properties that MUST occur in the Media Overlay Document;
the metadata element is provided for custom metadata requirements.
body ElementThe 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.
body
The body element is the REQUIRED second child of the smil element.
epub:type
[optional]
An expression of the structural semantics of the corresponding element in the EPUB Content Document.
The value is a white space separated list of property [[!Packages32]] types. Refer to Semantic Inflection for more information.
id
[optional]
The ID [[!XML]] of the 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]].
In any order:
At least one par or seq is REQUIRED.
seq ElementThe seq element contains media objects which are to be rendered sequentially.
seq
One or more seq elements MAY occur as children of the body element and of the seq element.
epub:type
[optional]
An expression of the structural semantics of the corresponding element in the EPUB Content Document.
The value is a white space separated list of property [[!Packages32]] types. Refer to Semantic Inflection for more information.
id
[optional]
The ID [[!XML]] of the 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]].
Refer to Structure for more information.
In any order:
At least one par or seq is REQUIRED.
par ElementThe par element contains media objects which are to be rendered in parallel.
par
One or more par elements MAY occur as children of the body and seq elements.
epub:type
[optional]
An expression of the structural semantics of the corresponding element in the EPUB Content Document.
The value is a white space separated list of property [[!Packages32]] types. Refer to Semantic Inflection for more information.
id
[optional]
The ID [[!XML]] of the element, which MUST be unique within the document scope.
In any order:
The audio element is OPTIONAL only if its
sibling text element refers to audio or
video media (see Embedded Media), or to textual content
intended for rendering via Text-to-Speech (TTS).
text ElementThe 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 Media).
text
As a REQUIRED child of the par element.
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 the element, which MUST be unique within the document scope.
Empty
audio ElementThe audio element represents a clip of audio media.
audio
A REQUIRED child of the par element unless
its sibling text element refers to audio
or video media, or to textual content intended for rendering via Text-to-Speech
(TTS), in which case it is OPTIONAL (see Embedded
Media).
id
[optional]
The ID [[!XML]] of the 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 Type Resources [[!EPUB32]] table.
clipBegin
[optional]
A clock value that specifies the offset into the physical media corresponding to the start point of an audio clip.
MUST be a [[!SMIL3]] clock value.
clipEnd
[optional]
A clock value that specifies the offset into the physical media corresponding to the end point of an audio clip.
MUST be a [[!SMIL3]] clock value.
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.
Empty
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 [[SMIL3]] 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.)
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.
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.
par elements can also be added to seq elements to define more complex
structures such as parts and chapters (see Structure).
In this section, the EPUB Content Document is assumed to be an XHTML Content Document. While Media Overlays can be used with SVG Content Documents, playback behavior might not be consistent and therefore interoperability is not guaranteed.
The body of a Media Overlay Document consists of
two elements: the par element and the seq element. The ordering of these elements MUST
match the default reading order of the EPUB Content Document.
The par element represents phrases. Each element identifies a text and audio
component to sychronize during playback.
The seq element represents sequences. It is used to represent nested containers such
as sections, asides, headers, and footnotes. It allows the structure inherent in these
containers to be retained in the Media Overlay Document.
The seq element MUST contain an epub:textref attribute. The IRI [[!RFC3987]] value of this attribute
MUST reference the corresponding structural element in an EPUB Content Document. As
seq elements do not provide synchronization instructions, this attribute allows
a Reading System to still match the element to a location in the text.
The following example shows a Media Overlay Document with nested seq elements,
representing a chapter with both a section header and a figure.
<smil xmlns="http://www.w3.org/ns/SMIL" xmlns:epub="http://www.idpf.org/2007/ops" version="3.0">
<body>
<!-- a chapter -->
<seq id="id1" epub:textref="chapter1.xhtml#sectionstart" epub:type="chapter">
<!-- the section title -->
<par id="id2">
<text src="chapter1.xhtml#section1_title"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:23:23.84"
clipEnd="0:23:34.221"/>
</par>
<!-- some sentences in the chapter -->
<par id="id3">
<text src="chapter1.xhtml#text1"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:23:34.221"
clipEnd="0:23:59.003"/>
</par>
<par id="id4">
<text src="chapter1.xhtml#text2"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:23:59.003"
clipEnd="0:24:15.000"/>
</par>
<!-- a figure -->
<seq id="id7" epub:textref="chapter1.xhtml#figure">
<par id="id8">
<text src="chapter1.xhtml#photo"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:24:18.123"
clipEnd="0:24:28.764"/>
</par>
<par id="id9">
<text src="chapter1.xhtml#caption"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:24:28.764"
clipEnd="0:24:50.010"/>
</par>
</seq>
<!-- more sentences in the chapter (outside the figure) -->
<par id="id12">
<text src="chapter1.xhtml#text3"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:25:45.515"
clipEnd="0:26:30.203"/>
</par>
<par id="id13">
<text src="chapter1.xhtml#text4"/>
<audio src="chapter1_audio.mp3"
clipBegin="0:26:30.203"
clipEnd="0:27:15.000"/>
</par>
</seq>
</body>
</smil>
The reason for grouping structures like sections, 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 figure, turning off rendering of
page break announcements (see Skippability and
Escapability), or customizing the reading mode to suit structures such as
tables.
Media Overlay text elements' src
attributes refer to EPUB Content Document elements by their IDs [[XML]]. The granularity
level of the Media Overlay therefore depends on how the EPUB Content Document is marked up. If
the finest level of markup is at the paragraph level, then that is the finest possible level at
which Media Overlay synchronization can be authored. Likewise, if sub-paragraph markup is
available, such as [[HTML]] span elements representing phrases or sentences, then finer
granularity is possible in the Media Overlay. Finer granularity gives users more precise results
for synchronized playback when navigating by word or phrase and when searching the text, but
increases the file size of the Media Overlay Documents.
Any EPUB Content Document associated with a Media Overlay MAY contain embedded media such
as video, audio, and images. The Media Overlay text element MAY be used in such instances to reference the embedded media
by its ID [[!XML]] value.
When a text element references embedded media that contains audio, the audio sibling element is OPTIONAL.
Authors SHOULD avoid using scripts to control playback of referenced embedded EPUB Content Document media, as this might conflict with Media Overlays playback behavior.
This specification allows the use of Text-to-Speech
(TTS) in addition to pre-recorded audio clips. When a Media Overlay text element with no audio sibling element references an element
within the target EPUB Content Document, the contents of that referenced element MUST be
appropriate for rendering via TTS. For example, it could be a textual EPUB Content Document
element or contain a text fallback.
In order to express semantic inflections, the epub:type attribute [[!ContentDocs32]] MAY be attached to Media Overlay par, seq,
and body elements.
Values for the Media Overlay epub:type attribute are constrained identically to the
epub:type attribute in EPUB Content Documents. Refer to Semantic Inflection
[[!ContentDocs32]] for details.
The epub:type attribute facilitates Reading System behavior appropriate for the
semantic type(s) indicated. Examples of these behaviors are Skippability and Escapability and Note.
This specification adopts the vocabulary association mechanisms defined in Vocabulary Association [[!ContentDocs32]] unmodified. Terms from the default vocabulary MUST be used unprefixed in Overlay Documents.
Media Overlays MAY use additional vocabularies by defining them in the epub:prefix attribute on the root smil element.
Visual rendering information for the currently-playing EPUB Content Document element MAY be
expressed in the CSS Style Sheet using author-defined classes. These author-defined class names
SHOULD be declared in the Package Document metadata using the metadata properties active-class and playback-active-class. The class names are then discoverable by Reading
Systems.
The active-class and playback-active-class properties MUST NOT be used in
conjunction with a refines attribute
[[!Packages32]], as they are always considered to apply to the entire Rendition.
This example demonstrates how Authors can associate style information with the currently-playing EPUB Content Document.
Although this example uses the class names -epub-media-overlay-active and
-epub-media-overlay-playing, any class names are permitted. The class names
chosen can be used along with any supported CSS features.
In this example, the Reading System would apply the author-defined
-epub-media-overlay-active class to each text element in the EPUB Content Document
as it became active during playback. Conversely, the class name is removed when the element is no
longer active. The user would see each EPUB Content Document element styled with a yellow background
for the duration of that element's playback.
The Reading System would also apply the author-defined -epub-media-overlay-playing class
to the document element of the EPUB Content Document when Media Overlays playback begins. The class
name is removed when playback stops. In the case of an XHTML Content Document, the class name would
be applied to the html element. In the case of an SVG Content Document, it would be
applied to the svg element. The user would see all the inactive text elements turn gray
during Media Overlays playback. When playback stopped, the elements’ colors would return to their
defaults.
If an EPUB Content Document is wholly or partially referenced by a Media Overlay, then its
manifest
item element [[!Packages32]]
MUST include a media-overlay attribute. The attribute MUST reference the ID
[[!XML]] of the manifest item for the corresponding Media Overlay Document.
The media-overlay attribute MUST NOT be attached to manifest item
elements that do not reference EPUB Content Documents.
Manifest items for Media Overlay Documents MUST have the media type
application/smil+xml.
The Package Document MUST include the duration of the entire Rendition in a meta element with the duration property.
In addition, the duration of each EPUB Content Document with an associated Media Overlay MUST be
provided. The refines attribute
[[!Packages32]] is used to associate each duration declaration to the corresponding
manifest
item [[!Packages32]].
Authors also MAY include narrator information in the
Package Document, as well as author-defined CSS class names
to be applied to the currently-playing EPUB Content Document element.
The prefix media: is reserved by [[Packages32]] for the inclusion of these
properties in package metadata.
When an EPUB Reading System loads a Package Document, it MUST refer to the
Manifest
item elements'
media-overlay attributes to discover the corresponding Media Overlays for EPUB
Content Documents. Playback MUST start at the Media Overlay element which corresponds to the
desired EPUB Content Document starting point. Note that the start of an EPUB Content Document MAY
correspond to an element at the start or in the middle of a Media Overlay. When the Media Overlay
Document has finished playing, the Reading System SHOULD load the next EPUB Content Document (as
specified in the Package Document spine) and also load its corresponding Media Overlay
Document, provided that one is given.
Reading Systems MUST render immediate children of the body element in a sequence. A seq element's children MUST be rendered in sequence, and playback
completes when the last child has finished playing. A par element's children MUST be rendered in parallel (with each
starting at the same time), and playback completes when all the children have finished playing.
When the body element's last child has finished playing, playback of the Media
Overlay Document is done.
When presented with a Media Overlay audio element,
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. The following rules MUST be
observed:
If clipBegin is not specified, its value is assumed to be
"0".
If clipEnd is not specified, its value is assumed to be the full duration of
the physical media.
If clipEnd exceeds the full duration of the physical media, then its value
is assumed to be the full duration of the physical media.
User-controllable audio playback options SHOULD include timescale modification, in which the playback rate is altered without distorting the pitch. The suggested range is half-speed to double-speed.
When presented with a Media Overlay text element,
Reading Systems SHOULD ensure the EPUB Content Document element referenced by the
src attribute is visible in the Viewport. 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 to the appropriate
elements in the EPUB Content Document. Conversely, the class names SHOULD be removed when the
playback state changes, as described in Associating Style
Information.
The active-class and playback-active-class metadata properties are
OPTIONAL, and if omitted, Reading System behavior is implementation-specific.
An EPUB Content Document with which a Media Overlay is associated MAY itself contain embedded video and audio media, which MAY be pointed to by Media Overlay elements. Unlike text and images, video and audio media has an intrinsic duration. Consequently, when a Reading System renders the synchronization described by a Media Overlay, the default playback behavior of audio and video media embedded within the associated EPUB Content Document MUST be overridden.
Note that the rules below apply only to referenced [[!HTML]] video or audio elements within the associated EPUB Content Document. That is to
say, the rules apply to only those elements pointed to by text elements within the Media Overlay (i.e., via the src
attribute). Embedded media that is not referenced by Media Overlay elements is not subject to
these rules.
All referenced audio and video media embedded within an EPUB Content Document MUST have their public playback interface deactivated (typically: play/pause control, time slider, volume level, etc.). This behavior is necessary to avoid interference between the scheduled playback sequence defined by the Media Overlay, and the arbitrary playback behavior due to user interaction or script execution. As a result, when the Reading System is in playback mode, it SHOULD:
Hide the individual video/audio UI controls from the page, which overrides the
default behavior defined by the [[!HTML]] controls attribute.
Prevent scripts embedded within the EPUB Content Document from invoking the JavaScript audio/video playback API (i.e., authored as part of the default behavior). It is RECOMMENDED that content producers avoid publishing embedded scripts dedicated to controlling the playback of embedded audio/video media. The published Media Overlay can then retain full control of the synchronized presentation without any risk of interference from script-enabled custom behaviors.
All referenced audio and video media embedded within an EPUB Content Document MUST be
initialized to their "stopped" state, and be ready to be played from the zero-position
within their content stream (possibly displaying the image specified using the [[!HTML]]
poster attribute). This requirement overrides the default
behavior defined by the [[!HTML]] autoplay attribute.
When an EPUB Content Document element becomes active, the CSS Style Sheet visual
highlighting rules apply regardless of the content type referred to by that element's
src attribute (e.g., the CSS class name defined by the active-class metadata property SHOULD be
applied to visible video and audio player controls within the host EPUB Content
Document).
In addition to the default behavior of Media Overlay activation for textual fragments and images, audio and video playback MUST be started and stopped according to the duration implied by the authored Media Overlay synchronization (as per the standard [[!SMIL3]] timing model). There are two possible scenarios:
When a Media Overlay text element has no audio sibling within its par parent container, the
referenced EPUB Content Document audio or video media MUST play until it ends,
at which point the text element's lifespan terminates. In this
case, the implicit duration of the text element (and by inference,
of the parent par container) is that of the referenced audio or
video clip.
When a Media Overlay text element has an audio sibling
within its par parent container, the playback duration of the
referenced EPUB Content Document audio or video media MUST be constrained by the
duration of the audio sibling. In this case, the actual duration of
the parent par container is that of the child audio clip,
regardless of the duration of the video or audio media pointed to by the
text element. This behavior can result in embedded video or
audio media ending playback prematurely (before reaching its full duration), or
ending before the playback of the parallel Media Overlay audio is
finished (in which case the last-played video frame SHOULD remain visible until
the parent par container finally ends). This behavior is equivalent
of the Media Overlay audio element implicitly carrying the behavior
of the [[!SMIL3]] endsync attribute.
Furthermore, Reading Systems SHOULD expose user controls for the volume levels of
each independent audio track (i.e., from the audio element of the
Media Overlay, and from the embedded audio or video media within the EPUB
Content Document), so that audio output can be adjusted to match listeners'
requirements. Note that having overlapping audio tracks is typically an
authoring-time concern: content producers usually add a layer of audio
information over a video track for description purposes. It is RECOMMENDED that
overlapping audio situations are carefully examined and dealt with at production
stage, as Reading Systems are NOT REQUIRED to handle simultaneous volume levels
in any particular way.
When a text element becomes inactive in the Media Overlay, and when it
points to embedded video or audio media, that referenced media MUST be reset to its
initial "stopped" state, ready to be played from the zero-position within their content
stream (possibly displaying the poster image specified using the [[!HTML]] markup).
When a Media Overlay text element with no audio sibling element references text within the
target EPUB Content Document, Reading Systems capable of Text-to-Speech (TTS) SHOULD render the
referenced text using TTS.
As per Reading System conformance requirements, the speech-related information provided in the target EPUB Content Document SHOULD be used to play the audio stream as part of the Media Overlay rendering. See Reading System Text-to-Speech Conformance Requirements [[!EPUB32]].
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).
While reading, users might want to turn on or off certain features of the content, such as
footnotes, page numbers, or other types of secondary content. This feature is called
skippability. Reading Systems SHOULD use the semantic information provided by Media Overlay
elements' epub:type attribute to
determine when to offer users the option of skippable features.
The following non-exhaustive list represents terms from the [[!EPUB-SSV]] for which Reading Systems might offer the option of skippability:
footnote
endnote
pagebreak
Reading System support for skippability based on epub:type values SHOULD NOT be
assumed.
Escapable items are nested structures, such as tables and lists, that users might wish to skip over, continuing to read from the point immediately after the nested structure. The escapability feature differs from the skippability feature in that it does not enable or disable entire types of items, but provides an exit from them (e.g., a user can listen to some of the content before choosing to escape).
Reading Systems SHOULD allow escaping of nested structures. Reading Systems MUST determine the
start of nested structures by the value of the epub:type attribute and SHOULD offer users the option to skip playback
of that structure and resume with whatever content comes after it.
The following non-exhaustive list represents terms from the [[!EPUB-SSV]] for which Reading Systems might offer the option of escapability:
table
table-row
table-cell
list
list-item
figure
The schema for Media Overlays is available at https://github.com/w3c/epubcheck/tree/master/src/main/resources/com/adobe/epubcheck/schema/30/media-overlay-30.nvdl.
Validation using this schema requires a processor that supports [[!NVDL]], [[!RelaxNG-Schema]], [[!ISOSchematron]] and [[!XMLSCHEMA-2]].
The NVDL schema layer can be substituted by a multi-pass validation using the embedded RELAX NG and ISO Schematron schemas alone.
The following are examples of allowed clock values:
5:34:31.396 = 5 hours, 34 minutes, 31 seconds and 396 milliseconds
124:59:36 = 124 hours, 59 minutes and 36 seconds
0:05:01.2 = 5 minutes, 1 second and 200 milliseconds
0:00:04 = 4 seconds
09:58 = 9 minutes and 58 seconds
00:56.78 = 56 seconds and 780 milliseconds
76.2s = 76.2 seconds = 76 seconds and 200 milliseconds
7.75h = 7.75 hours = 7 hours and 45 minutes
13min = 13 minutes
2345ms = 2345 milliseconds
12.345 = 12 seconds and 345 milliseconds