SignalingStateChange event is being fired as a result of setLocalDescription / setRemoteDescription.
But there are cases where signaling state will not be changed as a result of setLocalDescription / setRemoteDescription, for example.

pc.createOffer()
pc.setLocalDescription()
onSignalingStateChange() -- Event fired as a result of signaling state change
// Again if createOffer is called for second time.
pc.createOffer()
pc.setLocalDescription()
// There is no change in the signaling state but still onSignalingStateChange() event is firing.

Spec should modify text in setLocalDescription step 7 from


Fire a simple event named signalingstatechange at connection.

to 

Fire a simple event named signalingstatechange at connection, if signaling state is changed.