Excerpt from section 4.2.4 from EPUB Publications 3.0:

<package …
prefix="foaf: http://xmlns.com/foaf/spec/
dbp:  http://dbpedia.org/ontology/">
…
</package>

Excerpt from an unnumbered (sic) section of EPUB3 Fixed Layout Documents:

Implementors should note that future revisions of [Publications30] may establish the vocabulary represented by the URI http://www.idpf.org/vocab/rendition/# as a reserved vocabulary. In this case, the result will be that a) explicit mapping declaration using the prefix attribute will no longer be applicable, and b) the prefix ‘rendition’ will be reserved for this vocabulary. Future revisions of [Publications30] may also integrate the properties defined here into the Package Document default vocabulary. In this case the properties defined herein will be allowed to occur in Package Documents without a prefix.

In other terms, an attribute prefix="foobar: http://www.idpf.org/vocab/rendition/#" is needed on the package element for the time being to be able to parse <meta property="foobar:orientation">. But we're already warned that that foobar may become rendition w/o the need to declare the corresponding prefix on the package element. And well, even rendition could be itself dropped in the future. So in the long run, it will probably be <meta property="orientation"> w/o property prefix or prefix declaration.

<troll>I think it is not complex enough. Prefixes should be declared by extra meta elements themselves using a prefixed property, just to be able to have a few circular references...</troll>

Hey guys, why don't you finish shaking the shaker and get rid of the rendition prefix for good right now? Just for the record, I have in front of me here two EPUB3 documents, the first one uses the rendition property prefix without prefix declaration, the second uses the rendering property prefix instead of rendition but since the URI is ok, this is valid. In other terms, it's a mess already... I'm sure many epub3 Readers will look for rendition and only rendition, failing to correctly catch and resolve the prefix at all. Of course, the situation will get worse if the prefix becomes reserved or even integrated ! Pfff....