Hum, yet another bug in an EPUB3 spec... Excerpt (verbatim, nothing added or removed) from section 2.1.3.1.3 of EPUB Content Documents 3.0:

The prefix attribute

The prefix attribute definition is unchanged, but the attribute is defined to be in the namespace http://www.idpf.org/2007/ops when used in Content Documents.

Unchanged from what?!?