Nice to see that so well documented: I'd only seen the one most common reference, RFC 793. That said, it's still unclear how far this extends: the example give

Nice to see that so well documented: I'd only seen the one most common reference, RFC 793. That said, it's still unclear how far this extends: the example given is of an unknown error code, which might lead you to think that the requirement is "syntactically well-formed input where you can't 100% determine the semantics." That's a far cry from the way browsers handle malformed HTML. Similarly, you have to apply some judgment concerning what an agent can interpret the meaning of.