I basically agree with this, but what it comes down to is that "must" refers to the definition of the breed, and "should" refers to the ideals. As Christie says--the bulldog first must look like a bulldog. One could go down the philosophical road of why or what is the essence or origin of Bulldog. It's clear that Bulldogs were not the only dogs used for bullbaiting, but none of the others would meet the definitions intended by the "musts".
The semantic use of these terms is less important than understanding what it is that essentially a bulldog is and must be, as opposed to other breeds of dogs, or creatures in general.
That's what I would like to read more about: First the "musts" the essence of the Bulldog, and then the equally important "shoulds" --how that essence is developed in actual dogs (moving toward ideal)
I know little, but would like to learn more of those things.