Of not, will need to clarify the representation of http headers like this:-
should it be represented as
headernameA: value1, value2
MultivaluedMap is just a sub type of Map<String, List<String>> and it adds some utility methods like getFirst(K key).
Other than that, MultivaluedMap doesn't give any other serialization semantics as mentioned in the report. As far as HTTP is considered, both forms are equivalent. API doesn't have to prefer one form over the other.
I raised the possibility of using the JAX-RS multvaluedmap for headers on the expert group, it seems the sentiment is that for the couple of utility methods, its not worth the websocket spec building a dependency on that API. So we will keep the headers as they are.