normative polish

index.bs

@@ -1171,7 +1171,7 @@ The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "S
 
 : <dfn>Relying Party Identifier</dfn>
 : <dfn>RP ID</dfn>
-:: In the context of the [=web authentication api|WebAuthn API=], a [=relying party identifier=] is a [=valid domain string=] identifying the [=[WRP]=] on whose behalf a given [=registration=] or
+:: In the context of the [=web authentication api|WebAuthn API=], a [=relying party identifier=] MUST be a [=valid domain string=] identifying the [=[WRP]=] on whose behalf a given [=registration=] or
     [=authentication|authentication ceremony=] is being performed. A [=public key credential=] can only be used for
     [=authentication=] with the same entity (as identified by [=RP ID=]) it was registered with.