Anyone who truly wants to engage with the challenge posed by [the popularization of tablets] and be a part of evolving the medium of the “book” needs to take seriously the notion of digital-first content development. Being digital first means refusing to make the ebook version of your content an afterthought. The digital format of the book should not be merely a postproduction conversion of print-bound manuscript files; nor should it be an ex post facto “enhanced” version of content that has already been completed— for example, adding in a few video clips at the end of an otherwise static text. From conception to publication, true digital-first content is designed to take full advantage of the capabilities offered by dedicated ereaders, tablets, and smartphones to create something that is native to the platform.

There is no other source content format better suited to the task of developing digital-first content for the diverse ecosystem of ereading devices than HTML5, because you can develop directly for the browser (ereader software engines are effectively specialized Web browsers). As I argue in “The Case for Authoring and Producing Books in (X)HTML5,” authoring in HTML5 makes it “trivial to integrate…digital-first elements directly into the manuscript”

Source: HTML5 is the Future of Book Authorship, The key advantages of the HTML5 platform for authors and publishers by Sanders Kleinfeld, September 19, 2013