Harald:
I think that both MUST and SHOULD requirements ought to be included in interoperability testing. However, I think that MAY is going too far. Consider the following:You're probably right. But it still leaves the question of whether these features are specified enough for interoperable implementations hanging. Which is not a Good Thing.By the same logic, all the MAYs must be implemented or removed too.I think this is raising the bar above current practice.
For one example of such features being tested:
http://www.ietf.org/IESG/Implementations/http1.1-implementations.txt
For instance, Accept-Ranges: is not a MUST or SHOULD feature.