NULL-canonicalizer will also be useful. Canonicalization algorithm must not be able to manipulate content. NULL is fine, others might not be. Canonicalization algorithm must be included into the signature. Iffy proposition to allow different Canonicalization-Algorithms with different security properties.
Programmers make mistakes like that too.
This is a slippery slope, maybe better move into separate documents. Worth while replicating some of that info in the outside structure. Brown-Draft not a product of the WG, but possible to get there. RB presents his proposal. Originator info might offer different ways.