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Not as much as you think for basic fiction style epubs ...
Take a epub2 that passes epubcheck, and make a Checkpoint using Sigil. Then pass it through both the ePub3-itizer and Access-Aide. Make sure to use Sigil's epub3 tools to update the ncx from the Nav, update any manifest properties.
Make sure it passes epubcheck again. Then run a Checkpoint compare against the epub2 checkpoint you made.
You will see the Nav replacing the NCX, different metadata in the OPF, improved semantics, use of aria-roles and alt attributes for images (general accessibility improvements, etc).
You can then extend this with new tags such as audio, and video tags, sections, aside, etc, You can use full CSS 3. You can include smil files to sync audio and video, you can use javascripts, etc. You can set and use extended semantics....
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