http://www.intertwingly.net/wiki/pie/PaceMustUnderstandElement
Any Atom document MAY contain a single atom:must-understand element, which, if it appears, MUST be the first child element of the document element.
I think we need to add language stating that aggregators aggregating entries containing elements or attributes from a mU namespace MUST mark that namespace mU in the aggregate feed. This would be somewhat easier to do if this element appeared in atom:head (so that it could be in entry/head).
Each atom:ns element MUST contain only text, with no child elements, which is to be interpreted as an XML namespace name [XMLNS].
I'd be in favor of a slight change from this:
<ns>http://example.com/private/27</ns>
to either of these:
<ns>http://example.com/private/27</ns> <ns localnames="foo bar">http://example.com/private/27</ns>
The first would be as currently defined. The second of which would mean that the consuming application needn't understand the entire extension, but just the foo and bar elements or attributes. This would enable more applications to process feeds that they are capable of processing; and would protect against cases where an application understands one draft of an extension, but not things added in another draft without changing the namespace. Regardless of whether the extension's namespace SHOULD change in such a case, there's no way for us to be certain whether extension authors will change it or not.
