EPUB (short for electronic publication) is a free and open e-book standard by the International Digital Publishing Forum (IDPF). Files have the extension .epub.

EPUB is designed for reflowable content, meaning that an EPUB reader can optimize text for a particular display device. EPUB also supports fixed-layout content. The format is intended as a single format that publishers and conversion houses can use in-house, as well as for distribution and sale. It supersedes the Open eBook standard.

The EPUB 3.0 Recommended Specification was approved on 11 October 2011. EPUB 3.0 supersedes the previous release 2.0.1 of EPUB. Detailed descriptions of the differences between 3.0 and 2.0.1 can be found on the IDPF website.

EPUB 3 consists of a set of four specifications:

  •     EPUB Publications 3.0, which defines publication-level semantics and overarching conformance requirements for EPUB Publications
  •     EPUB Content Documents 3.0, which defines profiles of XHTML, SVG and CSS for use in the context of EPUB Publications
  •     EPUB Open Container Format (OCF) 3.0, which defines a file format and processing model for encapsulating a set of related resources into a single-file (ZIP) EPUB Container.
  •     EPUB Media Overlays 3.0, which defines a format and a processing model for synchronization of text and audio

The EPUB 3.0 format is intended to address the following criticisms:

While good for text-centric books, EPUB may be unsuitable for publications that require precise layout or specialized formatting, such as comic books. Also, it has been criticized for trying to solve an already solved problem instead of fixing unsolved problems.

A major issue hindering the use of EPUB for most technical publications is the lack of support for equations formatted as MathML. They are currently included as bitmap or SVG images, precluding proper handling by screen readers and interaction with computer algebra systems. Support for MathML is included in the EPUB 3.0 specification.

Other criticisms of EPUB are the specification’s lack of detail on linking into, between, or within an EPUB book, and its lack of a specification for annotation. Such linking is hindered by the use of a ZIP file as the container for EPUB. Furthermore, it is unclear if it would be better to link by using EPUB’s internal structural markup (the OPF specification mentioned above) or directly to files through the ZIP’s file structure. The lack of a standardized way to annotate EPUB books could lead to difficulty sharing and transferring annotations and therefore limit the use scenarios of EPUB, particularly in educational settings, because it cannot provide a level of interactivity comparable to the web.