EPUB is the dominant open standard for electronic books and other digital publications. The specification states that “EPUB defines a distribution and interchange format for digital publications and documents,” and that “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.” In practice an EPUB file is a ZIP archive: it bundles a set of Web documents, their stylesheets and images, a package document describing the publication’s metadata and reading order, and a navigation document that serves as the table of contents.
The defining characteristic of EPUB is that content is reflowable. Because each chapter is XHTML styled with CSS rather than a fixed-page image, a reading system can re-lay the text to fit any screen size, change the font, adjust margins, or enlarge type for accessibility, and the book simply re-paginates. This is the central distinction from PDF, which preserves a fixed page geometry. EPUB also supports a fixed-layout mode for content such as illustrated children’s books and comics where exact placement matters, along with media overlays for synchronized audio and embedded fonts and scripting.
EPUB grew out of the Open eBook Publication Structure and was released as EPUB by the International Digital Publishing Forum (IDPF) in 2007, with the major EPUB 3 revision following in 2011. EPUB 3 rebased the content format on HTML5 and modern CSS, replacing the older XHTML and proprietary pieces of EPUB 2 and aligning the standard with the Open Web Platform. In 2017 the IDPF merged into the World Wide Web Consortium, and stewardship of EPUB passed to the W3C, which placed the format on its standards-track process.
That process produced EPUB 3.3, which the W3C published as a W3C Recommendation on 25 May 2023, the first time the EPUB specification series reached full Recommendation status. EPUB 3.3 maintains backward compatibility with the widely deployed EPUB 3.2 while tightening conformance requirements and updating references to current Web standards. The companion EPUB Accessibility specification was advanced alongside it, reflecting the standard’s emphasis on making digital books usable by readers with disabilities.
EPUB’s importance is that it made the digital book a first-class member of the Web platform rather than a separate proprietary silo. By building publications out of the same HTML, CSS, and SVG that power web pages and wrapping them in a ZIP container, EPUB let any publisher produce books that render consistently across e-readers, tablets, and phones from many vendors. It is supported by the great majority of dedicated e-reading devices and apps and underpins much of the world’s commercial and library ebook distribution.