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Is epub3 an official universal open standard? Yes!
Has epub3 got enthusiastic supporters? Yes
Will epub3 work on any OS? Yes, but only a compatible app. Some apps have some subset of epub3, a few do all of it.
Is it universal on physical ereaders? Many don't support it at all. Some support a subset.
What about the main Bookstores - Gadget combos?
Amazon has about 92% of English Language. The azw3/KF8 is sort of their version of epub2 and conversion back & forth and appearance is similar. Their newer KFX does also support Sticky Note annotation and Print Replica. But it's not related to epub3. Most of the source of azw3 and KFX is epub2 (or epub2 fall back in epub3) or docx.
Use epub2 and dual mobi (a special format that has old mobi and azw3 in one file) on your website and everyone can read it.
You can put an epub2 system contents in an epub3 and ensure that the ebook "works" on an epub2 app or ereader. The epub3 adds extra features such as maths that textbooks and technical works need. It allows creation of fixed layout rather than reflowable (but that usually needs a 10″ or larger tablet and an epub3 app). It allows multimedia and interactive (often an app using a free framework is better).
It has more support for semantics in HTML (which is controversial except for epub3 evangelists) which might affect screen readers or conversion programs (no change to appearance, all can be done in epub2).
Is it universal per se? Only if you think IPv6 is Universal.
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