Consistency: Zero, 0, and non-zero

1. It's unclear why "zero" is preferred over "0" throughout the WD. For example, both styles are present in this sentence:

867 If the Will Flag is set to 0, the Will QoS and Will Retain fields in the Connect Flags MUST be set to zero

2. Both "non zero" and "non-zero" are present in the document. I'd prefer non-zero.