We have multiple attributes that we defines as "open enumerations" (NMTOKEN with predefined values).
I suggest to add a note to all those lists that requests implementors to report values they use that are not defined in the spec.
I see two advantages:
- it helps us to hamnoize the values used in the wild by providing suggestionsprovi to the naming and including them in the list
- it shows implementors how easy it can be to participate in the specification work