EPUB 3, maintained by the W3C, is the standard for reflowable digital books. It is fundamentally a packaged website: a ZIP archive containing HTML5, CSS3, SVG, and multimedia.
The EPUB 3 Package Structure
A valid EPUB contains a strict internal hierarchy:
mimetype: Must be the first file in the ZIP, uncompressed, containing exactly `application/epub+zip`.META-INF/container.xml: Points the reader to the root OPF file..opf(Open Packaging Format): The manifest listing every file in the book, the reading order (spine), and the metadata..ncxornav.xhtml: The structural table of contents.
Reflow vs. Fixed-Layout
While designed for reflowable text, EPUB 3 introduced Fixed-Layout (FXL) specifications. FXL EPUBs use CSS absolute positioning to lock text and images to exact coordinates, effectively turning EPUB into a web-based PDF alternative for comics and children's books.