The use of Websockets as a transport is described in WD13 Appendix A line 1711
Ken Borgendale suggests that this might also specify the close code and reason.
Java J2EE 1.7 and Mozilla javascript give the ability to set these but other interfaces may not., so we can only make a recommendation.
Normal closure ( 1000) would be the default, but should we recommend using 1002 Protocol_Error if there is an MQTT Protocol error etc?
What reason text should be sent if any?