Hi Antoine,
Thanks for submitting the issue. The WG discussed this and decided to close this issue with no action.
The compliance section of every specification states that an optional element of a request MUST be able to be handled by a server, unless the specification has specific text allowing it as a server option. Hence, the service is required to support the Content attribute.
The WG felt that since the specification currently states [1] that when the Content attribute is not present it defaults to any content type. This allows a client to NOT use the Content attribute that would allow the service to send back the metadata in any content type/format. Hence, there is no need for a fault for the service to indicate it does not support a
particular content type. Thus, a profile could remove the use of Content attribute by a client allowing a service to return any content type.
Further, if the client wants a particular content type or format, it can request that by specifying that in the GetMetadata request.
Please let us know if you have any questions.
Thank you.
[1] "When not present the default value is "http://www.w3.org/2002/ws/ra/edcopies/ws-mex/Content/Any".