OK here's what I have now in my working copy of rfc3920bis:
***
12.3. Well-Formedness
There are two varieties of well-formedness:
o "XML-well-formedness" in accordance with the definition of "well-
formed" in Section 2.1 of [XML].
o "Namespace-well-formedness" in accordance with the definition of
"namespace-well-formed" in Section 7 of [XML-NAMES].
The following rules apply.
An XMPP entity MUST NOT generate data that is not XML-well-formed.
An XMPP entity MUST NOT accept data that is not XML-well-formed;
instead it MUST return an <xml-not-well-formed/> stream error and
close the stream over which the data was received.
An XMPP entity MUST NOT generate data that is not namespace-well-
formed. An XMPP server SHOULD NOT route or deliver data that is not
namespace-well-formed, but MUST NOT return a stream error in response
to the receipt of such data.
Note: Because these restrictions were underspecified in an earlier
revision of this specification, it is possible that
implementations based on that revision will send data that does
not comply with the restrictions; an entity SHOULD be liberal in
accepting such data.
***
