The Accessible Publishing Knowledge Base was originally developed as a complement to the O'Reilly Accessible EPUB 3 ebook published in 2012. It was a way to surface all the information that was collected as part of authoring that book but that could not all be added to it.
The first iteration of the site was developed by DAISY and hosted on the (now retired) International Digital Publishing Forum (IDPF) web site — the organization that was originally responsible for the creation and maintenance of the EPUB standard. The hybrid guidelines also came packaged as an EPUB publication.
When the IDPF merged with the World Wide Web Consortium (W3C), DAISY took over hosting of the site and continued its role as maintainers of the content. The knowledge base has since grown to cover an extensive range of topics on creating accessible digital publications. While still primarily rooted in making EPUB 3 publications accessible, the site is also a source of accessible practices for EPUB 2 and also provides practical guidelines for creating accessible HTML content in W3C Audiobooks.
The intervening years have also seen the publication of the EPUB Accessibility specification, which defines a formal framework for checking the accessibility of EPUB 3 publications. This site is now a primary source of information for producing content to meet this specification.