A common mistake is to move short-form annotations with the dfdl: prefix from elements into the dfdl:format annotation. But then one forgets to remove the prefix dfdl: since the attributes of the dfdl:format annotation are not prefixed. So this is an error:
When what was intended is
Daffodil should issue a specific diagnostic whenever it finds dfdl: prefixed attributes in the annotation elements dfdl:format, dfdl:element, dfdl:simpleType, dfdl:sequence, dfdl:choice, etc.
It should point out specifically that the author probably wanted to remove the prefix dfdl: