As MotorCortex is a very general-purpose library it can’t predict what the initial value of an animated attribute for a given element might be.
That’s why Incidents implement the getScratchValue method. This method returns the scratch (initial) value of the element’s attribute. The value might be retrieved by the DOM itself, calculated or hard coded by any library. Whatever the getScratchValue returns that is going to be the initial value of the element’s attributeKey.