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 technical reports index at
https://www.w3.org/TR/.
W3C recommends the wide deployment of this specification as a standard for
the Web.
A W3C Recommendation is a specification that, after extensive
consensus-building, is endorsed by
W3C and its Members, and
has commitments from Working Group members to
royalty-free licensing
for implementations.
Future updates to this 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 which the individual believes contains
Essential Claim(s)
must disclose the information in accordance with
section 6 of the W3C Patent Policy.
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 should also 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.
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 algorithm explanations are 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 must keep track of the changes to HTML, and the technologies it
references, to ensure they keep their systems up to date.
This specification does not require EPUB reading systems to support scripting, 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 must 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 may 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-33] 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 may 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 information not required by this specification (e.g.,
package document metadata). Reading systems may use this additional information for any
purposes (e.g., to improve the user interface).
2.2 Error handling
Reading systems are not required 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 are not required 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.
EPUB creators, for example, can greatly benefit from having access to such processing information
(e.g., to efficiently debug their EPUB publications). For maximum use in debugging, it is
recommended 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.
It is recommended that reading systems support at least one of
the H.264 [h264] and VP8 [rfc6386] video codecs, but this is not a conformance requirement
— a reading system may support any video codec, or none at all. Reading system developers
should 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, and if a foreign
resource is not supported, MUST process fallbacks as defined in foreign resources [epub-33].
To limit the risk of network attacks, reading systems SHOULD only load remote resources referenced
via the https URI scheme [rfc9110].
3.4 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.
3.5 File URLs
Reading Systems MUST prevent access to
resources referenced via file URLs [rfc8089].
Furthermore, a reading system MUST also process the dir attribute [epub-33] 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-33]).
Refer to a resource's formal specification for more information about to handle the absence of
explicit language or direction information.
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).
3.9 External links
When a link has an http or httpsscheme [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.
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.
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.
Although EPUB creators are required to follow various file name and file path restrictions
[epub-33] for maximum interoperability, reading systems SHOULD attempt to process file names
and paths that do not adhere to these requirements. Invalid file names and paths may only be
problematic on some operating systems.
This specification does not specify how a reading system that is unable to represent OCF file
names and paths would 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-33] 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.
Reading systems MUST NOT use ancillary manifest information contained in the ZIP archive
or in the manifest.xml file [epub-33] for processing an EPUB publication.
Signature file (signatures.xml)
Before computing the digest used to validate the signature in the signatures.xml
file [epub-33], 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-33].
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.
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
Reading systems
SHOULD support deobfuscation of fonts as defined in Font
obfuscation [epub-33].
To restore the original data, reading systems should 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 may return a
valid font. Reading systems do not have to support this method of retrieval, but developers
should consider it when supporting EPUB 3 content generally.
If the dir attribute [epub-33] 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
Although setting the directionality of package document
metadata is marked as under-implemented in
[epub-33], 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.
The under-implemented label will be removed from [epub-33] when the necessary baseline of
support in reading systems has been achieved.
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, may require inspecting other metadata, such as their last
modification dates, titles, and authors.
To determine whether the value of a dc:identifier element [epub-33] conforms to an
established system or has been granted by an issuing authority, reading systems SHOULD check
for an identifier-type property
[epub-33].
The dc:title element
Reading systems MUST recognize the first
dc:title element [epub-33] 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)
When determining display
priority, reading systems MUST use the document order of dc:creator elements [epub-33]
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-33] 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-33] value, it SHOULD treat the value of
the element as a string.
The language identified in an hreflang [EPUB-33] 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.
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.).
A reading system that does not support the MIME media
type [rfc2046] of a given publication resourceMUST traverse the manifest fallback chain [epub-33] until it
identifies a supported publication resource to use in place of the unsupported resource. If the reading system supports multiple
publication resources in the fallback chain, it MAY select the resource to use based on the
resource's properties attribute
[epub-33] values, otherwise it SHOULD honor the EPUB creator's preferred
fallback order.If a reading system does not support any resource
in the fallback chain, it MUST alert the user that it could not display the content.
When manifest fallbacks [epub-33] are provided
for top-level content documents, reading systems MAY choose from the available options to find
the optimal version to render in a given context (e.g., by inspecting the properties attribute for
each).
A reading system MUST terminate the fallback chain at the
first reference to a manifest item it has already encountered.
5.5 Spine
Reading systems MUST provide a means of
rendering an EPUB publication in the order defined in the spine element [epub-33], which
includes:
rendering successive primary items in the order given in the spine.
When a user traverses the default reading order defined in
the spine element, a reading system MAY automatically skip non-linear itemref elements
[epub-33]. 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 EPUB creator has not specified the page-progression-direction attribute [epub-33], 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-paginatedXHTML content documents.
Reading systems MUST ignore all
values expressed in spine itemrefproperties attributes [EPUB-33] that
they do not recognize.
Reading systems MUST NOT skip spine references to duplicate manifest items [EPUB-33] 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.
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-33] in reading systems is OPTIONAL. Reading systems
MUST ignore collection elements that define unrecognized roles.
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.
For the purposes of
styling SVG embedded in XHTML content documentsby 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.
6.1.2.4 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.
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 6.4 Scripting.
MUST, if it has a viewport,
support the visual rendering of SVG using CSS as defined in Styling [svg] and it SHOULD support all properties defined in the
Property Index [svg]. In the case of embedded SVG,
a reading system MUST also conform to the constraints defined in 6.1.2.3.1 Embedded SVG and CSS.
SHOULD NOT override the EPUB creator's style sheets, but SHOULD do
so in a way that preserves the Cascade when necessary: through a user agent style sheet, or
[html] style attributes.
It MAY override parts of the EPUB creator's style sheet because
of user interaction.
In addition to supporting CSS properties as defined above, a reading
system's user agent style sheet SHOULD support the [html] suggested
default rendering.
Reading system developers should implement CSS support at the level of major browsers and publicly
document their user agent style sheets and how they interact with EPUB creator's style sheets.
Reading systems SHOULD support spine-level scripting in
reflowable EPUB content documents that use the "scrolled-doc" or "scrolled-continuous" [epub-33] presentation modes defined by the rendition:flow property. Similarly, if it
supports spine-level scripting in reflowable EPUB content documents, it MUST implement
the "scrolled-doc" presentation mode and SHOULD implement the
"scrolled-continuous" presentation mode.
Reading systems MAY support scripting in other
contexts, but this specification does not address such scripting. As a result, the use of
scripting in these contexts may not be consistent across reading systems.
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.4.1.2 Features in container-constrained scripting
contexts.
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.
6.4.2 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 must ensure that scripts cannot disable critical functionality (such as
navigation) to constrain the extent to which a potentially malicious script could impact their reading systems. As a result, although
the scripting environment should be able to cancel the default action of any event, some events
either might not be passed through or might not be cancelable.
6.4.3 Security considerations
This section is non-normative.
Reading system developers who also support scripting must be aware of the security issues that
arise when reading systems execute scripted content. As the underlying scripting model employed
by reading systems and browsers is the same, developers must take into consideration the same
kinds of issues encountered in web contexts.
Each reading system must establish if it can trust the scripts in a particular document. Reading
systems should treat all scripts as untrusted (and potentially malicious), and developers should
examine all vectors of attack and protect against them. In particular, developers should
consider the following:
an attack against the runtime environment (e.g., stealing files from a user's hard
drive);
an attack against the reading system itself (e.g., stealing a list of a user's books or
causing unexpected behavior);
an attack of one 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).
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")
may maintain the publication's unique origin when the publication is removed and subsequently
re-imported into the content library. Conversely, reading systems may create a new unique origin
for every newly added publication.
This specification also recommends that container-constrained scripts [epub-33] not be allowed to modify the DOM of the host
EPUB content document and/or manipulate the containing rectangle (see 6.4 Scripting).
Note that compliance with these recommendations does not guarantee protection from the possible
attacks listed above; developers must examine each potential vulnerability within the context of
their reading system.
SHOULD, when
generating non-HTML based navigation widgets, replace unsupported non-text elements in headings
and labels with their alternative text [epub-33].
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-33] is present.
MAY provide users access to the links in the landmarks nav element [epub-33] and
MAY use the links to enable functionality in the application.
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 [csssnapshot].).
Reading systems MUST
honor the above requirements irrespective of whether the EPUB navigation document is part of the spine.
When the
rendition:layout property is set to pre-paginated, reading
systems MUST NOT include space between the adjacent content slots when rendering synthetic spreads.
The rendition:layout property values have the following processing
requirements:
reflowable
Reading systems MAY apply dynamic pagination when rendering.
pre-paginated
Reading systems MUST produce exactly one page per spine itemref [epub-33] when
rendering.
The rendition:spread property values have the following processing
requirements:
none
Reading systems MUST NOT incorporate spine items in a synthetic spread. Reading
systems SHOULD create a single viewport positioned at the center of the
screen.
landscape
Reading systems SHOULD render a synthetic spread for spine items only when the device
is in landscape orientation.
portrait (deprecated)
Reading systems SHOULD treat the value "portrait" as a synonym of
"both" and create spreads regardless of orientation.
both
Reading systems SHOULD render a synthetic spread regardless of device
orientation.
auto
Reading systems MAY use synthetic spreads in specific or all device orientations as
part of a content display area utilization optimization process.
The rendition:page-spread-left and rendition:page-spread-right
properties apply to both pre-paginated and reflowable content, and they only apply when the
reading system is creating synthetic spreads.
Reading systems MUST honor
rendition:page-spread-* properties on both reflowable and pre-paginated
spine items (e.g., by inserting a blank page).
When a pre-paginated
spine item follows a reflowable one, the pre-paginated one SHOULD start on the next page (as
defined by the page-progression-direction attribute) when it lacks a
rendition:page-spread-* property value.
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.
8.1.2 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-33].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 may:
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.
When the ICB aspect ratio does not match
the aspect ratio of the reading system content display area, reading systems should
follow the rules for the viewBox and the preserveAspectRatio attributes, as defined in [SVG].
8.1.3 Viewport rendering
When rendering fixed-layout documents, the default intent is that the content display areaSHOULD 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.
If a reading system supports the specified rendering, it SHOULD use that method to handle
overflow content, but MAY provide the option for users to override the requested rendering.
The default global value is auto if no meta element carrying this
property occurs in the metadata
section [epub-33]. Reading systems MAY support only this default value.
The rendition:flow property values have the following processing requirements:
paginated
The reading system SHOULD dynamically paginate all overflow content.
The reading system SHOULD render all EPUB content documents such that users can scroll
overflow content, and SHOULD present each spine item as a separate scrollable
document.
auto
The reading system MAY render overflow content using its default method or a user
preference, whichever is applicable.
For the rendition:flow-scrolled-continuous property, the scroll direction MUST be
defined relative to the block flow direction of the root element of the XHTML content document referenced by the itemref element [epub-33]. The scroll direction MUST be
vertical if the block flow direction is downward (top-to-bottom). It MUST be horizontal if the
block flow direction of the root element is rightward (left-to-right) or leftward
(right-to-left).
Reading system developers may decide to disregard this restriction, and accept the
scrolled-continuous value of rendition:flow as a switch to display
each pre-paginated spine item on a long, vertical strip (making it is easier to read on a
smartphone or computer). This type of presentation is often referred to as "webtoons". Some
publishers already use this possibility. After some further experimentation and incubation, a
future version of this specification may introduce this approach as a standard feature for
fixed-layout documents. See also the (deferred) github issue 2412.
8.2.2 The rendition:align-x-center property
When the rendition:align-x-center property is set on a
spine item, reading systems SHOULD render the content centered horizontally within the
viewport or spread, as applicable. This property does not affect the rendering of the spine
item, only the placement of the resulting content box.
For reflowable content,
reading systems that support this property MUST center each virtual page.
This specification does not define a default rendering behavior when reading systems do not
support this property or EPUB creators do not specify it. Reading systems MAY render spine
items by their own design.
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 Basic playback
9.2.1 Timing and synchronization
Reading systems MUST render immediate children of the body element [epub-33] in a
sequence. A seq element's [epub-33] children MUST be rendered in sequence, and playback
completes when the last child finishes playing. Reading system MUST render a par element's
[epub-33] children in parallel (with each starting at the same time), and playback completes
when all the children finish playing. Reading system playback of the media overlay document
completes when the body element's last child finishes playing.
9.2.2 Rendering audio
When presented with a media overlay
audio element [EPUB-33], reading systems MUST play the audio resource referenced by the
src attribute, starting at the clip offset time given by the clipBegin attribute and ending
at the clip offset time given by the clipEnd attribute [epub-33].
In addition:
If the EPUB creator has not specified a clipBegin attribute, reading
systems MUST assume the value "0".
If the EPUB creator has not specified a clipEnd attribute, 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.
9.2.3 Rendering EPUB content document elements
When presented with a media overlay text element [epub-33] 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.
The active-class and playback-active-class metadata properties are
OPTIONAL, and if omitted, reading system behavior is implementation-specific.
Reading system behavior when a fragment
identifier [epub-33] does not reference an element is also
implementation-specific.
9.3 Interacting with the EPUB content document
Note
Earlier versions of this specification included some information about embedded audio
and video [epubmediaoverlays-32]. This feature has been deprecated.
9.3.1 Navigation
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.
EPUB creators may associate media overlay document elements with EPUB content document
structures such as tables. Reading systems should ensure that media overlay playback remains
synchronized with user navigation of table rows and cells. The reading system might also
play the corresponding table header preceding the contents of the cell.
9.3.2 Embedded audio and video (deprecated)
Guidance for automatic playback of embedded audio and video is now deprecated.
When a media overlay
text element with no audio [epub-33] sibling element references text within the target
EPUB content document, reading systems capable of text-to-speech (TTS) playback SHOULD
render the referenced text using TTS.
Reading systems SHOULD use the speech-related information provided in the target EPUB content
document to play the audio stream as part of the media overlay rendering.
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 should treat the duration property
values set in the package document as approximative when making use of them.
9.4 Skippability and escapability
9.4.1 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.
9.4.2 Escapability
While playing media overlays, reading systems SHOULD offer users
the option to leave ("escape") escapable
structures [epub-33], which are determined by the presence of an epub:type
attribute [epub-33] 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.
This section defines an algorithm for obtaining an expanded URL from a property data type
value. It applies only to reading systems that support the [epub-33] vocabulary association
mechanisms.
Reading systems that do not support the [epub-33] vocabulary association mechanisms MAY process
property values as plain string values [infra]. It is not required to support the
expansion of these values to URLs to 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 property value to process.
attr: the name of the attribute that contains value.
elem: the name of the element that attr is attached to.
To obtain an expanded URL, apply the following steps:
Let baseURL, expandedURL, propertyPrefix, and
propertyReference be empty strings.
Explanation
In this algorithm:
baseURL will hold the base URL associated with the value, whether this URL is
assigned by the EPUB creator, 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 property reference.
propertyPrefix will hold the property's prefix [epub-33] (i.e., the value
before the colon). Properties from a default vocabulary will not have a prefix, but all
others will.
propertyReference will hold the property's reference [epub-33] (i.e., the
value after the prefix). All valid properties require a reference.
Obtain the values of propertyPrefix and propertyReference as follows:
If value does not contain a colon (U+003A), set propertyReference
to value.
Explanation
If a property 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 propertyPrefix to
the string before the colon and set propertyReference to the string after the
colon.
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 must not 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 must be percent encoded.
Obtain the value of baseURL as follows:
If propertyPrefix is an empty string:
If attr is epub:type [epub-33], set baseURL to
http://idpf.org/epub/vocab/structure/#
Otherwise, if elem is the package document meta element [epub-33]
and attr is scheme, the property value is invalid. Return null.
Otherwise, if elem is the package document meta element [epub-33],
set baseURL to
http://idpf.org/epub/vocab/package/meta/#
Otherwise, if elem is the package document link element [epub-33],
set baseURL to
http://idpf.org/epub/vocab/package/link/#
Otherwise, if elem is the package document item element [epub-33],
set baseURL to
http://idpf.org/epub/vocab/package/item/#
Otherwise, if elem is the package document itemref element
[epub-33], set baseURL to
http://idpf.org/epub/vocab/package/itemref/#
Explanation
If the property value 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
property 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-33]. 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
propertyPrefix, set baseURL to the URL associated with
the prefix declaration.
Otherwise, if propertyPrefix matches a reserved prefix [epub-33]
for doc, set baseURL to the URL associated with the
reserved prefix.
Explanation
When a property value 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
property data types as defined in [epub-33].
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
propertyReference. Do not add a separator when concatenating the values.
Although an expanded value may be obtained, it is not necessarily the case that it is a valid
URL. This is only likely to occur when the EPUB creator assigns an invalid base URL 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 property data type value to a full
URL only ensures a reading system has encountered the value it expects (i.e., because different EPUB
creators can assign the same prefix to different URLs, two values may appear the same until
expanded).
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.
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 must offer. This does not mean
that there are not common accessibility issues that all reading systems developers should be aware of,
or seek to avoid in their applications.
The W3C's User Agent Accessibility Guidelines [uaag20] provides many useful practices developers should
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. Activating the
table of contents links should be possible using different inputs.
The DAISY Consortium maintains an accessibility test suite
to aid in evaluating these issues and more.
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.
The greatest threats to users come from the content they
read [epub-33], 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 must take into
consideration, with further details and recommendations in the following sections.
Scripting
Malicious scripts present several attack vectors against reading systems. If local storage is
not secure, for example, they can attempt to compromise the user's data. Provided with
network access, they may attempt unauthorized collection of user data.
EPUB publications may contain resources designed to exploit security flaws in reading
systems or the operating systems they run on. Attackers may 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 may be
compromised.
Calls to remote resources can also be used to track information about users (e.g., through
server logs). Reading systems should limit the information they expose through HTTP requests
to only what is essential to obtain the resource.
The origin of an EPUB is both unknown to the EPUB creator and specific to each reading system implementation. Consequently, if the EPUB
creator hosts remote resources on a web server they control, the server effectively cannot
use security features that require specifying allowable origins, such as headers for CORS, Content-Security-Policy, or X-Frame-Options.
External links
Like remote resources, external links may be used to trick unwitting users into opening
malicious resources on the web designed to exploit the reading system or operating
system.
Likewise, reading systems should also avoid exposing potentially identifying information
about users through the traversal of links (e.g., not using tracking identifiers or exposing
unnecessary information about the user's environment).
User-generated content
User-generated content within a reading experience, for example through text areas or other
interactive components, could pose a threat to reading system security or user privacy if
the content is not properly secured. More details on scripting security are provided in 6.4.3 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.
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 may be required for the sale, delivery, and
operation of an EPUB publication, particularly on platforms where the sale of an EPUB
publication and the method of reading it are connected. In these cases, the reading system SHOULD
identify the data being collected, how it is used, and allow for user opt-outs (retailers may choose
to inform users by other means, however, 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 may be required or helpful for some reading system affordances.
In these cases, anonymization is also RECOMMENDED. Reading systems also SHOULD 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.
Developers should 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.
This specification does not define an epubReadingSystem property extension for the
WorkerNavigator object [html]. Reading systems therefore do not have to expose the
epubReadingSystem object in the scripting context of Workers, and EPUB creators cannot rely on its presence.
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 implementations may create cloned instances of the epubReadingSystem
object in scripted content documents for technical feasibility reasons. In such cases, the
reading system must ensure they consistently maintain the object's state — as reflected by the
values of its properties and methods — across all copied instances.
The hasFeature method returns a
boolean value indicating whether the reading system supports any version of the specified
feature, or undefined if the reading system does not recognize the specified
feature.
The optional version parameter allows EPUB creators to query custom features
that could change in incompatible ways over time. The return value indicates support only
for the specified version of the feature.
Features defined in this specification are
versionless. If a reading system supports a feature defined in this specification, it MUST
ignore any supplied version parameter and return a true value.
B.4.1.2 Features
The
following table lists the set of features that reading systems that support the
epubReadingSystem object MUST recognize. When the features are queried from
the hasFeature method, reading systems MUST return a boolean value indicating
their support.
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.
01-Feb-2023: Removed reference to getOverrideStyle in DOM Level 2 as it was dropped
from the DOM Living Standard. See pull
request 2531.
11-Jan-2023: Added note advising reading system developers to support package metadata
directionality even though marked as under-implemented for authoring. See pull request 2515.
11-Jan-2023: Removed support description for under-implemented features. See pull request 2515.
06-Jan-2023: Reduced the requirement to support prefixed CSS properties to a recommendation. See
pull request 2514.
06-Jan-2023: Reduced the requirement to use alternative text for non-text elements in the
navigation document to a recommendation due to the lack of support for both. See pull request 2513.
05-Jan-2023: Removed the processing requirements for linked records. See pull request 2512.
11-Nov-2022: Added requirement to replace unsupported non-text elements with their text
alternatives when generating non-HTML navigation widgets. See issue 2477.
13-Oct-2022: Added sections to explain expectations for error handling and reporting. See issue 2454.
11-Oct-2023: Additional text sketching the RS behavior in the case of erroneous ICB setting. See
issue 2442.
06-10-2022: Moved the restriction on XML external identifiers from the XML conformance clause to
the security and privacy section, adding an additional note on security risks on internal or
external entities. See issue 2433
and issue 2477.
23-Sep-2023: Added a note on a possible future feature, whereby the value of
rendition:flow would control the publication of webtoon-like publications. See
issue 2412.
14-Sept-2022: Integrated the reading system support requirements for unsupported features that
were still in the EPUB 3.3 specification. See issue 2424.
28-Aug-2022: The guidelines for rendering embedded timed media in Media Overlays have been
deprecated. See issue 2397.
28-July-2022: Restructured the vocabulary association mechanisms section as an algorithm for
obtaining an expanded URL. See issue
2378.
22-July-2022: The name and version properties of the
epubReadingSystem object have been deprecated. See issue 1872.
19-July-2022: ZIP containers with content split into segments have been disallowed in general.
See issue 2360.
24-June-2022: Removed duplicate requirement not to use hreflang value for
processing purposes. See pull request
2343.
17-June-2022: Add recommendation to alert user before opening URLs with a scheme and clarify the
external links section to account for more than just web schemes. See issue 2320.
17-June-2022: Clarify support for the vocabulary association mechanisms is optional. See issue 2317.
07-June-2022: The usage of File URLs has been disallowed. See issue 2324.
07-June-2022: Noted that unsigned EPUB publications represent a security risk and added
recommendation to treat sideloaded unsigned publications as untrusted. See issue 2265.
07-June-2022: Updated privacy and security recommendations to use normative language.
27-May-2022: Added reading system conformance section. See issue 2271.
27-May-2022: Added recommendation to only load remote resources referenced via https. See issue 2263.
20-May-2022: Removed the recommendation to prevent access to persistent storage from "unrelated
documents". This recommendation conflicts with, and is made redundant by, the unique origin
requirement introduced in this revision. See issue 2264.
17-May-2022: Added an index of terms. See issue 2260.
31-Mar-2022: Moved custom attribute authoring requirements to the authoring specification. Added
reading system handling of custom properties. See issue 2134.
17-Mar-2022: Added requirement to suppress playback of skippable elements in media overlays
Documents. See issue 2066.
11-Feb-2022: Fix the contradictory support statements for media overlays. Support is recommended
when rendering of prerecorded audio is supported, as in previous versions of EPUB 3. See issue 1991.
04-Feb-2022: Changed the non-normative security considerations for controlling network access,
opening external links, and managing local storage to normative recommendations. See issue 1957.
04-Feb-2022: Expanded the section on security and privacy to include new sections on the threat
model for reading systems and additional recommendations for ensuring security and privacy. See
issue 1871, issue 1872, issue 1875 and issue 1876.
03-Feb-2022: Reduced the requirement to support deobfuscation to a recommendation. See issue 1873.
19-Jan-2022: Elevated default statements for the rendition:spread and
rendition:orientation properties to requirements, to align with
rendition:layout. See issue 1936.
08-Dec-2021: page-spread-center is now an alias for spread-none. See
issue 1929.
10-Nov-2021: Proper definition of the content URL and handling of relative URLs. See issue 1374 and issue 1888.
01-Nov-2021: Added a statement on the reading system behavior when item references are repeated
in the spine. See issue 1686.
25-Oct-2021: Clarify that reading systems must render non-linear resources when they are
hyperlinked to by a user. See issue
1864.
27-Aug-2021: Added accessibility bullet to cover the need for zooming, particularly for
fixed-layout documents. See issue
1776.
27-Aug-2021: Remove the recommendation to support text search and selection in SVGs. Replacing
with a more general bullet on search accessibility to avoid defining requirements on reading
system UI. See issue 1774.
09-July-2021: Restored the custom attributes section due to known usage but without reference to
the attribute registry. See issue
1602.
09-July-2021: Removed recommendation to use the first language tag to determine an unspecified
page progression direction. See issue
1482.
20-May-2021: Providing a shared and unique origin for scripts is a requirements for reading
systems that support scripting. See issue 1659.
20-May-2021: Removed requirement for reading systems to be conformant xml:base processors. See
pull request 1678.
12-May-2021: Added additional requirement that reading systems ignore property values that
expand to invalid URL strings and property values with empty references. See issue 1673.
12-May-2021: Changed all references to URIs/IRIs to URLs and references to RFCs 3986 and 3987 to
the URL standard. See issue 808.
06-May-2021: Recommend that reading systems support references to HTML target elements and SVG
fragment identifiers in textref attributes and the text element's
src attribute. Support for other fragment identifier schemes is optional. See
issue 1586.
23-Apr-2021: Clarified that reading systems should attempt to process invalid file and path
names. See issue 1629
09-Apr-2021: Added a new section dedicated to accessibility and removed the outdated reference
to the defunct appendix in the Accessibility specification. See issue 1608.
09-Apr-2021: Added section clarifying all the conformance requirements on establishing the
primary language and base direction of publication resources. See pull request 1613.
01-Apr-2021: Added section clarifying how absolute URLs are created from relative in the package
document. See pull request 1468.
30-Mar-2021: Restructured the document to remove the rendundant conformance sections and
requirements that were only used as locators when this specification was combined with the
authoring requirements. See pull request
1597 and pull request
1609.
23-Mar-2021: Added requirement to prevent top-level navigation to data URLs. See issue 1564.
23-Mar-2021: Changed "suppressing" of non-linear content to "skipping" when traversing the spine
to clarify that the intention is not to remove all access to such content. See issue 1480.
10-Mar-2021: Changed restriction against using resources not listed in the package document to a
recommendation not to use. See issue
810.
09-March-2021: the statement on unique identifiers has been set to non-normative, following the
discussion and Working Group resolution. See issue 1310.
08-Mar-2021: Remove unnecessary requirement to assume default value for rendering metadata. See
issue 1313.
05-Mar-2021: Added requirement for reading systems to collapse whitespace in DCMES and meta
elements per the [infra] specification definition. See issue 1295.
26-Feb-2021: Clarified the trimming of leading and trailing whitespace is to be done in
accordance with the [infra] specification definition, and removed the exception that
meta element properties may define their own whitespace handling rules. See issue 1295.
19-Feb-2021: Updated the scripting security
considerations so that the text recommends assigning a unique origin to each EPUB
publication for security rather than domain isolation at the EPUB content document level. See issue 1156.
15-Feb-2021: Clarified the requirements for presenting nav elements in the navigation document.
The toc nav is now the only required element, the page-list nav is recommended when present, and
all others are optional. See issue
975.
05-Nov-2020: Generalized the restriction on custom attribute namespace URIs to exclude all of
idpf.org and w3.org. See issue
1388.
30-Sept-2020: The structure of the EPUB core specifications has been simplified to ease
understanding and access to information. This specification now consolidates the reading system
requirements from the EPUB 3.2 specification together with the Packages, Content Documents, OCF
and media overlays specifications. The EPUB 3.3 core specification now focuses only on authoring
requirements.
E. Acknowledgements
This section is non-normative.
Specifications, like art, are human creations. No human has done more for EPUB than Garth Conboy, who has
been there every step of the way, from the very first OEB 1.0 in 1999 to today's EPUB 3.3. None of this
would have happened without Garth's vision, knowledge, and preternatural good nature. We dedicate EPUB 3.3 to his memory.
We are forever in your debt, Garth.
The following members of the EPUB 3 Working Group contributed to the development of this specification:
Shadi Abou-Zahra (Amazon)
Will AWAD (Newgen Knowledgeworks)
Noel Ray Barron (Apple Inc.)
Sofia Bautista (Legible Media Inc.)
Leah Brochu (National Network for Equitable Library Service)
Matthew C. Chan (House of Anansi Press)
Yu-Wei Chang (Taiwan Digital Publishing Forum)
Juan Corona (Legible Media Inc.)
Dave Cramer (W3C Invited Expert, chair)
Romain Deltour (DAISY Consortium)
Marisa DeMeglio (DAISY Consortium)
Brady Duga (Google LLC)
Reinaldo Ferraz (NIC.br - Brazilian Network Information Center)
Symon Flaming (Rakuten Group, Inc.)
John Foliot (W3C Invited Expert)
Teenya Franklin (Pearson plc)
Hadrien Gardeur (EDRLab)
Matt Garrish (DAISY Consortium)
Jen Goulden (Crawford Technologies)
David Hall (Apple Inc.)
Ivan Herman (W3C, staff contact)
Tetsu Hoshino (Kodansha, Publishers, Ltd.)
jasen huang (ByteDance)
Norikazu Ishizu (Kadokawa Corporation)
Norihito IYENAGA (Kodansha, Publishers, Ltd.)
Rick Johnson (VitalSource Technologies)
Ken Jones (Circular Software)
Antonio Kamiya (Dentsu Group Inc.)
Deborah Kaplan (W3C Invited Expert)
Bill Kasdorf (Book Industry Study Group)
Hiroshi Kawada (MEDIA DO Co., Ltd.)
George Kerscher (DAISY Consortium)
Kazuhito Kidachi (Mitsue-Links Co., Ltd.)
Masakazu Kitahara (Voyager Japan, Inc.)
Toshiaki Koike (Voyager Japan, Inc.)
Ryo Kuroda (ACCESS CO., LTD.)
Charles LaPierre (Benetech)
Dan Lazin (Google LLC)
Philippe Le Hegaret (W3C)
Laurent Le Meur (EDRLab)
YuYu Lin (Taiwan Digital Publishing Forum)
Farrah Little (National Network for Equitable Library Service)
Victor Lopes (Apple Inc.)
Karan Malhotra (Newgen Knowledgeworks)
Makoto Murata (DAISY Consortium)
Cristina Mussinelli (Fondazione LIA)
Yoichiro Nagao (Kodansha, Publishers, Ltd.)
Theresa O'Connor (Apple Inc.)
Yoshinori Ohmura (SHUEISHA Inc.)
Rachel Osolen (National Network for Equitable Library Service)