I'm working on my first 3.0 epub and having trouble with listing ncx in the opf as a backup for older readers. The idpf EPUB Validator says
Spine item with non-standard media-type 'application/x-dtbncx+xml' with no fallback
I have this in the manifest
<item id="ncx" href="toc.ncx" media-type="application/x-dtbncx+xml"/>
and this in the spine
<spine>
<itemref idref="cover" linear="no"/>
<itemref idref="book" linear="yes"/>
<itemref idref="toc" linear="no"/>
<itemref idref="ncx" linear="no"/>
</spine>
Thanks!