JAVASERVERFACES-2349 asks to make Mojarra conform to this spec text, In section "State Saving Alternatives and Implications" in the "Application Integration" chapter.
The default implementation Serializes the view in both the client and server modes.
This issue has a long history, dating back to summer 2007, chronicled in
JAVASERVERFACES-632. In that issue, then implementation lead Ryan Lubke stated that he polled the community and the result was that enforcing serializability in all cases should be off by default.
At this point, I'd like to amend the text quoted above to be:
The default implementation Serializes the view in client mode. Serializing the view in server
mode is optional but must be possible by setting context-param javax.faces.SERIALIZE_SERVER_STATE