Originally Posted by user_none
The EUP 3 spec has been out (final form) for over a year now. People have been saying this since before the spec was finalizes. The fact of the matter is new readers have come out and they're not supporting epub 3.
As soon as ADE supports ePub 3 then everyone will need to. Until then ...
There are a few ePub 3 supported devices out there although most only have partial support, i.e. FK8 and iBooks. Others are connected to HTML 5 browsers such as Google.