I don't think we require anything. Countries are taking our guidelines and adopting them. If we say this is their problem, not ours, we are saying they need to change our wording. Whatever problems we leave in, they must take out. But W3C has gone on record against modifications to our specs.
Hard requirements are also important for machine checkability.
All things we do have to be measurable. I'm not sure they have to be machine measurable. They have to be if we want them to be automatable. Automtation facilitates a lot of things. But even if the requirements are not automatable, if they can be objective, it would be good.