Digital publications are made up of many different files: HTML documents, images, CSS files and scripts, among others. In order to simplify distribution, the files are all bundled together in a single container file so users only have to download the one file to obtain the entire publication.

EPUB, for example, uses ZIP as its container format. EPUB files can be opened by any ZIP tools, an operation that is made even easier by changing their extension from .epub to .zip (i.e., most computer operating systems have built-in tools to open files with .zip extensions).

Another common name for the EPUB container file is the "OCF file" or "OCF Container", as the requirements for zipping the content files together is defined in the EPUB Open Container Format (OCF) specification.

Content Provider

The organization responsible for providing a digital publication to users, whether through an sale in online bookstore, by download from a corporate Web site, through a library lending program, or otherwise.

Core Media Type

In EPUB 3, a core media type is a file format that has to be supported to ensure that publications are readable in whatever reading system a user happens to use.

For example, EPUB 3 requires reading systems be able to render XHTML5 and SVG documents, MP3 and MP4 audio, a variety of image formats, etc. Therefore, these are all core media types.

If the media type of a resource is not listed as core, it is considered a foreign resource (i.e., reading systems might support it, but are not guaranteed to).

The list of core media types supported in EPUB 3 is provided in the EPUB 3 specification.


The Cascading Style Sheet (CSS) language is used to control the presentation of HTML and SVG documents.

CSS is used to specify fonts, colours, borders and other display aspects based on a set of rules defined by the author.

A common complaint with digital publications, however is that reading systems do not support, or ignore, author rules, or apply their own. The result is that publications often do not look as intended, and differ in appearance from reading system to reading system.