For the past ten months the open-source Epub creation tool Sigil has had an option to create Epub3 files, but only as a post-production conversion. Soon that will change.
The developers of Sigil have released a new version of the app which they say represents the first step towards better Epub3 support. It doesn't add support for editing Epub3 files, but it is a step in the right direction:
Today we are releasing Sigil-0.8.900 which really represents the first of the Sigil 0.9.0 pre-releases whose eventual goal is to add epub3 support without disrupting epub2 editing.
To accomplish this, Sigil has had most of its internals torn out and rebuilt from scratch ...
The developers went on to ask that anyone who installs the new release to please test it and report any irregularities they encounter, irregardless of scale. They want to iron out any difficulties ASAP before continuing development towards support for both Epub and Epub3.
You can download the pre-release Sigil from GitHub.
Let's hope they can get through this quickly. According to what I've heard on Twitter, the current method for converting from Epub to an Epub 3 file (an external plugin) can take up to a minute to generate an Epub3 ebook (which sometimes needs some tweaking before it will validate).
That's a workable solution, but it's not ideal. Once all the bigs are worked out, the new way to create an Epub3 file should be as simple as selecting "Save As".
image by Cuyahoga jco