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It can use more advanced semantic features that will help people with disadvantages. It can also hold audio and video legally (some epub2 try to extend with these). Furthermore, when properly created with guide and toc.ncx, then can easily be backwards compatible with epub2.
So the real question is why use still use "epub2" when you can create a fully backwards compatible "epub3" to support both markets and provide additional features if so desired and provide better semantic meaning support for disadvantaged readers.
Perhaps it is time to fully retire legacy "epub2" and work to make "epub4" something better for everyone.
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