Epub3 adoption has largely ground to a start (pun intended) since the ebook format was finalized 2 years ago. Aside from a handful of closed gardens, no reading platform has adopted more than a smidgen of Epub3's many features.
I had been hoping ever since the implementation project was announced in July that the publishers would get together and come up a limted subset of the Epub3 spec (I called it the Epub3Lite format), but that doesn't appear to have happened. Instead the list of features that publishers want most include such gems as MathML.
MathML (Mathematical Markup Language) is a way to write out equations. It's mostly
only used by a niche textbook and technical publishers, and it was added to the Epub3 spec at their request. Unfortunately, it is not easy to implement (not the way that the Epub3 spec requires), which is part of the reason few apps which support Epub3 also support MathML.
MathML was at the top of my list of components to subtract from Epub3Lite format; I wanted to make it optional and thus reduce the workload of all the developers who weren't going to implement it anyway. The fact that it is still included is a sign that Epub3 came out of the AAP project in almost the same form as it went in.
And that means, folks, that Epub3 is still as stalled today as it was in July. A couple dozen reading apps support a smidgen of Epub3 features, but only a handful offer anywhere even close to support for the full Epub3 spec.
Even Sony only has limited support for Epub3; they only went as far as audio, video, and some interactivity. And while there are many Android reading apps which mention Epub3 support, many don't promise support for anything more than audio and video (Moon+ Reader, for example).
At this point it seems that widespread adoption of Epub3 is still waiting on the Readium project. This project is going to produce an SDK which app developers will be able to use to build Epub3 reading apps (I hope).
The resulting apps won't be available for 6 months or more, though it is possible that Bluefire will get their Epub3 app out the door sooner. They're one of the lead developers on the Readium project.
P.S. If you have a moment, please direct your attention to the logo in the upper right corner. If you are restraining your juvenile giggles at the thought of the AAP's EPUBE project, you're not alone.