In the spec it says that a <diffuse> inside a <phong> (or <lambert>, etc) can be declared 0 or 1 times, however it lists the default value as “N/A”. If it doesn’t have to be declared then it needs a reasonable default (black probably), right? Same goes for the other material properties (<emission>, <ambient>, …).
I’ll submit a bug with Khronos if we agree this is an oversight.