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ePub is an open format defined by the Open eBook Forum of the International Digital Publishing Forum (<IDPF>). It is based on XHTML and XML along with optional CSS style sheets. Its predecessor was the OEB standard. Specifications are found at the IDPF web site.



Quoted from the IDPF web site:

"'.epub' is the file extension of an XML format for reflowable digital books and publications. '.epub' is composed of three open standards, the Open Publication Structure (OPS), Open Packaging Format (OPF) and Open Container Format (OCF), produced by the IDPF. '.epub' allows publishers to produce and send a single digital publication file through distribution and offers consumers interoperability between software/hardware for unencrypted reflowable digital books and other publications. The Open eBook Publication Structure or 'OEB', originally produced in 1999, is the precursor to OPS."


The intent of ePub is to serve both as a source file format and an end user format. For this reason the files are collected into a container for easy dissemination and use. This container is generally a zip file but the extension has been renamed to .epub. It has special requirements by including an uncompressed mime type file while the rest of the data in the file is compressed. An ePub reader should be capable of reading the content in its compressed format.

Mime Type: Multipurpose Internet Mail Extensions (http://www.isi.edu/in-notes/rfc2045.txt). “MIME media types” provide a standard methodology for specifying the content type of objects.


The IDPF specification page contains the specifications for this format. In particular check the version 2.0 OPS and OPF specifications and the version 1.0 OCF specifications . The informational documents are also quite useful in understanding the standard's intent and content.


A typical OCF is a zip file that might look like:

  Great Expectations.opf
     … other HTML files for the remaining chapters …


The first file in the ZIP Container MUST be a file by the ASCII name of ‘mimetype’ which holds the MIME type for the ZIP Container (i.e., “application/epub+zip” as a 20 character ASCII string; no padding, CR/LF, white-space or case change). The file MUST NOT be compressed nor encrypted and there MUST NOT be an extra field in its ZIP header.


The container.xml is a required file with a required name. It must be in the META-INF folder. All other folders are optional and can be any name the user chooses. The container.xml file shows the filename and location of the OPF file.


The Open Packaging Format (OPF) Specification, defines the mechanism by which the various components of an OPS publication are tied together and provides additional structure and semantics to the electronic publication.

Specifically, OPF:

  • Describes and references all components of the electronic publication (e.g. markup files, images, navigation structures).
  • Provides publication-level metadata. Specifically it should include: dublin core formatted data
  • Specifies the linear reading-order of the publication.
  • Provides fallback information to use when unsupported extensions to OPS are employed.
  • Provides a mechanism to specify a declarative table of contents (the NCX).
  • May provide pointers to additional optional elements such as embedded fonts.

An example:

<package version="2.0" xmlns="http://www.idpf.org/2007/opf"
    <metadata xmlns:dc="http://purl.org/dc/elements/1.1/"
          <dc:title>Alice in Wonderland</dc:title>
          <dc:identifier id="BookId" opf:scheme="ISBN">
          <dc:creator opf:role="aut">Lewis Carroll</dc:creator>
       <item id="intro" href="introduction.html"
               media-type="application/xhtml+xml" />
       <item id="c1" href="chapter-1.html"
               media-type="application/xhtml+xml" />
       <item id="c2" href="chapter-2.html"
               media-type=application/xhtml+xml" />
       <item id="toc" href="contents.xml"
               media-type="application/xhtml+xml" />
       <item id="oview" href="arch.png"
               media-type="image/png" />
    <spine toc="ncx">
       <itemref idref="intro" />
       <itemref idref="toc" />
       <itemref idref="c1" />
       <itemref idref="c2" />
       <itemref idref="oview" linear="no" />


The Open Publication Structure (OPS) Specification describes a standard for representing the content of electronic publications.


  • The specification is intended to give content providers (e.g. publishers, authors, and others who have content to be displayed) and publication tool providers, minimal and common guidelines that ensure fidelity, accuracy, accessibility, and adequate presentation of electronic content over various Reading Systems.
  • The specification seeks to reflect established content format standards.
  • The goal of this specification is to define a standard means of content description for use by purveyors of electronic books (publishers, agents, authors et al.) allowing such content to be provided to multiple Reading Systems and to insure maximum presentational equivalence across Reading Systems.


XHTML is predefined XML and as such it should begin with the line:

<?xml version="1.0" encoding="ISO-8859-1"?>

where the character set to be used in the book is defined in the encoding entry.

A conforming OPS document must support the following XHTML constructions.

XHTML 1.1 Module NameElements (non-normative)Notes
Structure body, head, html, title the default rendering for body is consistent with the CSS property page-break-before having been set to right (which behaves like always on one-page Reading Systems), but may be overridden by an appropriate style sheet declaration.
Text abbr, acronym, address, blockquote, br, cite, code, dfn, div, em, h1, h2, h3, h4, h5, h6, kbd, p, pre, q, samp, span, strong, var The optional attribute cite may be used in blockquote, q, del and ins to provide a URI citation for the element contents. Reading Systems are not required to process or use the referenced URI resource, whether or not the resource is listed in the Manifest.
Hypertext a Reading Systems may use or render a URI referenced physical resource not listed in the Manifest (i.e., it is not a component of the Publication), but they are not required to do so.
List dl, dt, dd, ol, ul, li
Object object, param The object element is the preferred method for generic object inclusion. When adding objects whose data media type is not drawn from the OPS Core Media Type list or which reference an object implementation using the classid attribute, the object element must specify fallback information for the object, such as another object, an img element, or descriptive text.
Presentation b, big, hr, i, small, sub, sup, tt
Edit del, ins
Bidirectional Text bdo
Table caption, col, colgroup, table, tbody, td, tfoot, th, thead, tr
Imageimg The inline element img should only be used to refer to images with OPS Core Media Types of GIF (http://www.w3.org/Graphics/GIF/spec-gif89a.txt), PNG (RFC 2083), JPG/JFIF (http://www.w3.org/Graphics/JPEG) or SVG (http://www.w3.org/TR/SVG11/). The required URI attribute, src, is used to reference the image resource, which must be listed in the Manifest.

The required alt attribute should contain a brief and informative textual description of the image. This text may be used by Reading Systems as an alternative to, or in addition to, displaying the image. The text is also an acceptable fallback for an img with src referencing a non-OPS Core Media Type for which no viable fallback was found in the manifest.

Client-Side Image Map area, map
Meta-Information meta
Style Sheet style The type attribute of the style element is required and must be given the value of text/css or the deprecated text/x-oeb1-css.
Style Attribute (deprecated) style attribute
Link link The link element allows for the specification of various relationships with other documents. Reading Systems must recognize external style sheet references specified via the href attribute and the associated rel attribute (for the values rel="stylesheet" and rel="alternate stylesheet".)
Base baseThe root of an ePUB file is the top of the file hierarchy inside the container.


Relationship to NVDL

This specification uses the NVDL language (see http://standards.iso.org/ittf/PubliclyAvailableStandards/c038615_ISO_IEC_19757-4_2006(E).zip) as a means to unambiguously define the interaction between the various schemas used in this specification. NVDL allows for interaction and validation between various XML schema languages. See Appendix A for a normative NVDL definition of OPS.

This specification does not require the use of NVDL tools to validate OPS documents, although such tools are available and may be used for validation.

Relationship to XHTML and DTBook

This specification recognizes the importance of current software tools, legacy data, publication practices, and market conditions, and has therefore incorporated certain XHTML 1.1 Document Type Modules and DTBook as Preferred Vocabularies. This approach allows content providers to exploit current XHTML and DTBook content, tools, and expertise.

To minimize the implementation burden on Reading System implementers (who may be working with devices that have power and display constraints), the Preferred Vocabularies do not include all XHTML 1.1 elements and attributes. Further, the modules selected from the XHTML 1.1 specification were chosen to be consistent with current directions in XHTML.

Any construct deprecated in XHTML 1.1 is either deprecated or omitted from this specification; CSS-based equivalents are provided in most such cases. Style sheet constructs are also used for new presentational functionality beyond that provided in XHTML.

Relationship to CSS

This specification defines a style language based on CSS 2. (Note that the CSS 2.1 specification is currently still at "Working Draft" status.) The style sheet MIME type text/x-oeb1-css has been deprecated in favor of text/css.

Relationship to XML

OPS is based on XML because of its generality and simplicity, and because XML documents are likely to adapt well to future technologies and uses. XML also provides well-defined rules for the syntax of documents, which decreases the cost to implementers and reduces incompatibility across systems. Further, XML is extensible: it is not tied to any particular type of document or set of element types, it supports internationalization, and it encourages document markup that can represent a document’s internal parts more directly, making them amenable to automated formatting and other types of computer processing.

  • Reading Systems must be XML processors as defined in XML 1.1. All OPS Content Documents must be valid XML documents according to their respective schemas.

Relationship to XML Namespaces

Reading Systems must process XML namespaces according to the XML Namespaces Recommendation at http://www.w3.org/TR/xml-names11/. For example:


Relationship to Dublin Core

Dublin Core is the defined standard for all metadata used in the ePub document. Only the id, title, and language are required but other enries are encouraged.


Checking tools


  • Info ZIP and 7zip can be used to pack and unpack the archive.
  • EPubHub - A tool to work with ePUB containers and modify the contents directly.
  • ePUB Fixer - quickly repair TOC and other modifications to an ePUB file.
  • Metadata editor - specific for ePUB files.


  • It is possible to make an eBook that conforms to the standard by placing the entire book contents in one XHTML file but the performance will be impacted by this decision. For best performance a standard size book should be divided into several files as the full file needs to be loading into memory at once. This is usually accomplished by separating the files by chapter.
  • Some mobile devices cannot handle large ePUB files. Generally this is caused by having an XHTML file that is too large. If the file can be expanded the large XHTML file may be able to be broken into multiple files. Typically the files should not exceed about 300K for best performance.
  • The ePub file format has proper support for TOC, through the use of TOC.NCX files. Most reader applications support this. This is documented in the DTBook standard.


ePUB 3

Here is some preliminary information on EPUB version 3

Thus far the specifications is headed for HTML5, CSS, SVG, images, multi-media support and will likely embrace MathML, SMIL and more.

For more information

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