Clients that do not preserve foreign markup when editing entries may cause data loss.
User uses a client that understands extension_A to create an entry containing extension_A
Same user comes back later using a different client that does not understand extension_A. As is allowed by RFC4287, the client ignores the unknown foreign markup. When the entry is PUT back, extension_A is silently dropped.
Both clients may be fully compliant with RFC4287 and the APP spec, and yet data is lost
At the very least, the spec should indicate that clients SHOULD preserve unknown foreign markup when editing.
(I'm confident that the specific wording can be improved)
Add the following:
8.2.1 Editing entries with foreign markup To avoid unintentional loss of data when editing entry collection members, APP clients SHOULD preserve all metadata, including unknown foreign markup, that has not been intentionally modified.