Ban non-IETF use of the Atom namespace.
Keep extensions in other namespaces, so the Atom namespace can be safely extended by the IETF.
6.1 Extensions From Non-Atom Vocabularies
This specification describes Atom's XML markup vocabulary. Markup from other vocabularies ("foreign markup") can be used in an Atom document, but MUST be namespace-qualified and in a namespace other than Atom's. Note that the atom:content element is designed to support the inclusion of arbitrary foreign markup.
Note: the following sections are unchanged from format-08. They are included for context.
6.2 Extensions To the Atom Vocabulary
Future versions of this specification could add new elements and attributes to the Atom markup vocabulary. Software written to conform to this version of the specification will not be able to process such markup correctly and, in fact, will not be able to distinguish it from markup error. For the purposes of this discussion, unrecognized markup from the Atom vocabulary will be considered "foreign markup".
6.3 Software Processing of Foreign Markup
Software processing an Atom Document which encounters foreign markup in a location that is legal according to this specification MUST NOT stop processing or signal an error. It might be the case that the software is able to process the foreign markup correctly and does so. Otherwise, such markup is termed "unknown foreign markup".
When unknown foreign markup is encountered as a child of atom:entry, atom:feed, or a Person construct, software MAY bypass the markup and any textual content and MUST NOT change its behavior as a result of the markup's presence.
When unknown foreign markup is encountered in a Text Contruct or atom:content element, software SHOULD ignore the markup and process any text content of foreign elements as though the surrounding markup were not present.