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Will EPUB 2 remain an active standard now that EPUB 3 is out?
More specifically:
Should all new EPUB development use EPUB 3 regardless of the target platform (meaning e-ink along with tablet)?
Are current e-ink devices such as the Sony PRS-*, Simple Nook, etc able to display EPUB 3 or should books for these devices continue to be EPUB 2?
EPUB 2 will probably not remain an active standard, in the sense that it would evolve over time. The EPUB 2 standard is of course there and stays there and there will be files and devices that follow that standard.
Looking at the differences between EPUB 3 and EPUB 2 it seems entirely feasible to make an EPUB 3 device (with or without multimedia) that also properly supports EPUB 2 files. This is little extra work and much more interesting for device manufacturers than asking for and implementing EPUB 2 extensions.
There is probably also going to be a subset of devices that don't support the EPUB 3 multimedia extensions, but are based on a rendering engine that supports EPUB 3. Whether you depict such a device as EPUB 2 with some EPUB 3 support or EPUB 3 with backwards compatibility is probably a marketing decision.
From a file generation point of view, you can include EPUB 2 compatible info in an EPUB 3 file. I expect publishers to do that. And if they don't it is feasible to add that information later on to such EPUB 3 file. I expect e.g. Calibre to do get such a feature, if it not already can do so. Of course that will not give you multimedia, but for normal books (with the same info as EPUB 2 files have now) that will work.
We've been creating epub 3.0 files for almost 2 years where I work. Those files have been sold through all the major retailers (though Kobo apparently stopped accepting epub 3.0 at some point a few months back, at least through Kobo Writing Life. That may have changed). As others have noted, the only thing you need to do if you want to read epub 3.0 files on an older device is include a little extra data--in particular, the toc.ncx file (which specifies the table of contents in epub 2.0.1) needs to be included in addition to the toc.xhtml file (which does the same job in epub 3.0). All of the other new markup in epub 3.0 will be happily ignored by older reading systems.
Long story short, there's no reason not to produce files in epub 3.0--it offers some neat new features (like much more comprehensive semantic markup with epub:type), and as more reading systems move to support it, you'll see a progressive enhancement of your book as those cool new features get utilized.
This was discussed in detail at the XML SummerSchool in Oxford last Sept [2013] and the consensus of experts and publishers was that EPUB3 is the way things are going, but some EPUB2 legacy is inevitable and needs to be managed.
As Anthon said, this may be done by embedding EPUB2 metadata into EPUB3 documents. However, most publishers are moving towards an automated (eg XSLT2) workflow, so creating both EPUB2 and EPUB3 versions at the same time is also possible, in order not to lose the market of existing EPUB2-only readership.
