BCP 70 " Guidelines for the Use of Extensible Markup Language (XML) within
IETF Protocols"
attempted to outline some of the design considerations for data representation
using XML.
In 2003, it represented the consensus and also the disagreements about what
was "best current practice" at the time.
Section 3 of BCP 70, "Alternatives", lists some of the alternatives and provides
a comparison.
I think what might be missing is an update to BCP 70 or a companion which more
explicitly compares XML, JSON, CBOR and other alternatives in use in IETF
protocols.
If you're interested in this work, perhaps you might review BCP 70 and suggest
updates.
Larry
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http://larry.masinter.net