The OPDS Catalog specification defines the namespace name for elements defined in the specification and used in OPDS Catalog Feed Documents and OPDS Catalog Entry Documents as:
http://opds-spec.org/2010/catalog
OPDS Catalog Feed Documents and OPDS Catalog Entry Documents MUST be "namespace-well-formed" as specified in Section 7 of Namespaces in XML 1.0 (Second Edition)].
This specification uses the prefix "opds:" for the namespace name, but this prefix is not semantically significant.