The widespread  adoption of EPUB as the standard format for eBooks took place  along with skyrocketing consumer demand for eBooks on dedicated eReader devices: Sony Reader, Amazon Kindle, Barnes & Noble Nook, Kobo, and many others. The version of EPUB powering this rise of eBooks, EPUB 2, was developed by the IDPF in 2006-2007 and matched up well with the capabilities of these dedicated devices. EPUB 2 only supports reflowable content, but these devices typically had small screens so fixed-layout content wasn’t practical. Their “E Ink” based screens didn’t support color or speedy screen redraw so video and interactivity weren’t even possible.  For the novels and linear non-fiction that are most appropriate for E Ink devices,  even more basic typographical niceties like custom fonts and fancy text styling weren’t necessarily critical (at least according to some reading system developers). EPUB 2 was based on an older version of HTML, only supported limited CSS styling, had no support for modern Web capabilities  like video and interactivity, and no requirement to handle embedded fonts but it met the demands of the supply chain at the time.

During 2010-2011 the IDPF developed the specification for EPUB 3. EPUB 3 modernizes EPUB by basing its content structure on the latest Web Standards including HTML5 and CSS3 Modules. EPUB 3 adds support for audio, video, embedded fonts, and the option of fixed-layout pages. EPUB 3 also supports global languages including vertical writing, left-to-right text and page progression, and other typographic features needed for Asian and other complex non-European writing systems. Last, but not least, EPUB 3 incorporates a number of important features to enable it to be a fully accessible format. EPUB 2 content is, generally, relatively accessible - more so than final-form representations like PDF - but EPUB 3 mainstreams features such as pronunciation hints and synchronization of pre-recorded audio with text display that were previously found only in specialized formats designed for use by people with print-disabilities. In addition, the base markup of EPUB 3 (HTML5) has enhanced semantics compared to the older version used by EPUB 2 (XHTML 1.1), with elements like section, article, figure, etc. EPUB 3 also specifies MathML, which makes mathematics markup more flexible and dynamic - a benefit to all readers - but also much more accessible to print disabled readers.  Finally, and perhaps most importantly, the inclusion of ARIA [link] markup allows complex interactive publications to become more accessible than before.

EPUB 3 matches up with the richer capabilities of tablets and larger-screen smartphones, which are rapidly becoming the most popular platforms for digital reading. However, given the large installed base of dedicated reading systems, and the millions of titles optimized for E Ink limitations, the transition from EPUB 2 to EPUB 3 has taken quite a while. Browser support for HTML5 has rapidly improved and since EPUB 3 implementations are by and large being built on top of the latest  browser engines (rather than via specialized rendering software as was common with EPUB 2) it’s reasonable to expect the EPUB 3 adoption situation to increase rapidly. Accelerating EPUB 3 support has been a key focus of the IDPF and a number of other stakeholders seeking to ensure widespread and full-fledged support for this key open standard.