The OEBZIP files were introduced by the International Digital Publishing Forum (IDPF). Likewise, OEB, these files are XML-based specification for the structure, content, and presentation of electronic books but it is in compressed or zip form.
Technical Details of OEBZIP File Format
The OEBZIP provided more new functionality in the area of rendering control, including, among other things, improvements in the markup vocabulary (now a basic subset of XHTML 1.1), and widely expanded CSS support. Sustainability factors are unchanged from the earlier version.
The OEBZIP file format consists of a set of files, including a compulsory package file, which includes a manifest listing all the other component files, and a spine, which indicates logical reading order. Moreover, to text in XML, a reader for OEBZIP files must be able to render images in JPEG and PNG formats. Content components in other formats may be incorporated, but fallback content must be provided in one of the basic OEB formats.
The OEBZIP bundle of files was used primarily as a mid-state format, with publication to end-users managed through publishers or aggregators who provided eBooks in forms suitable for different eBook viewers. Its successor version, EPUB is more likely to reach end-users in a final-state format, as well as serving as a mid-state format.