I updated the RSS profile to reflect the spec's rule against extending
core RSS elements with attributes in a namespace.

The Introduction section now includes this sentence:

"The elements defined in the specification MUST NOT be extended by
attributes defined in a namespace."

http://www.rssboard.org/rss-profile#introduction

The RSS 2.0 specification includes an Extending RSS section that
describes how the format can be extended by namespaces:

http://www.rssboard.org/rss-specification#extendingRss

"RSS originated in 1999, and has strived to be a simple, easy to
understand format, with relatively modest goals. After it became a
popular format, developers wanted to extend it using modules defined
in namespaces, as specified by the W3C.

"RSS 2.0 adds that capability, following a simple rule. A RSS feed may
contain elements not described on this page, only if those elements
are defined in a namespace."

Because this language mentions elements and excludes attributes, the
simplest read of the spec is that core elements cannot contain
namespaced attributes.

For those who are new to this list, the RSS Profile is a set of best
practices for RSS we've been working on since March. New drafts are
published for public comment -- if you agree or disagree with a
revision, please let us know here on RSS-Public.