[css3-3d-transforms] translateZ() transformation function and percentage values

The Editor's draft [1] says the function :

	#  specifies a translation by the given amount in the Z direction. 
	# Note that percentage values are not allowed in the translateZ translation-value, 
	# and if present are evaluated as 0.

I would expect a value that is not allowed to result in an invalid transform function and
thus the property being ignored. What is the intent of non-allowed percentage values to 0 ?


[1] http://dev.w3.org/csswg/css3-3d-transforms/