Since the consensus seems to be to change to "validation-methods",
here's the wording I propose for the validation-methods section:
Extensions to the CAA Record: validation-methods Parameter
A CAA parameter "validation-methods" is also defined for the "issue" and
"issuewild" properties. The value of this parameter, if specified, MUST
be a comma-separated string of challenge method names. Each challenge
method name MUST be either an ACME challenge method name or a
CA-assigned non-ACME challenge method name.
The presence of this parameter constrains the property to which it is
attached. A CA MUST only consider a property with the
"validation-methods" parameter to authorize issuance where the name of
the challenge method being used is one of the names listed in the
Where a CA supports both the "validation-methods" parameter and one or
more non-ACME challenge methods, it MUST assign identifiers to those
methods. These identifiers MUST be chosen to minimise the likelihood of
conflict with any ACME challenge method name; it is RECOMMENDED that, at
the very least, CAs avoid assigning identifiers ending in a hyphen and
two digits ("-00").
A CA SHOULD assign individual identifiers to each of its non-ACME
challenge methods. However, if it is unable or unwilling to do so, it
MAY use the fallback identifier of "non-acme" to identify such methods.
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