[sbazerque]

But "the signer" here is a cryptographic identity, that may be present in more than one device. So, even when conceptually it is just one entity, in practice it may be several computers doing something independently, and one may need the result to be the same given identical inputs.

[formerly_proven]

Offhand that sounds really cursed: Signing the "same" thing multiple times with the same key shared among multiple devices and also expecting the signatures to be identical. I honestly can't think of anything that would require something like this and it sounds like a really bad idea as well.

Just to clarify something I've seen a few siblings confused about: Saying your signature scheme should not rely on canonical representations doesn't mean you shouldn't parse the input properly before processing it (e.g. checking that the input corresponds to a schema). It just means those two things are separate things best done separately. It's a little like NATting and firewalling: often confused with each other, or people arguing one acts like the other, but they're really separate things, and everything is much easier once you understand and heed that.