allow UAs to not show the permission prompt

spec/index.bs

@@ -906,7 +906,7 @@ the exception thrown.
             {{IdentityCredentialRequestOptions/mode}} to customize the dialog.
         1. Otherwise, let |permission| be the result of running [=request permission to sign-up=]
             algorithm with |account|, |config|, |provider|, and |globalObject|. Also set
-            |permissionRequested| to true.
+            |permissionRequested| to true if the user agent [=supports showing a permission prompt=].
     1. Otherwise:
         1. Set |account| to the result of running the [=select an account=] from the
             |accountsList|.
@@ -1378,7 +1378,8 @@ an {{IdentityProviderAPIConfig}} |config|, an {{IdentityProviderRequestOptions}}
         metadata=] with |config|, |provider|, and |globalObject|.
     1. Prompt the user to gather explicit intent to create an account. The user agent MAY use the
         {{IdentityProviderBranding}} to inform the style choices of its UI. Additionally:
-        1. If |fields| is not [=list/empty=]:
+        1. The user agent SHOULD <dfn>support showing a permission prompt</dfn> as follows.
+        1. If |fields| is not [=list/empty=] and the user agent [=supports showing a permission prompt=]:
             1. If |metadata| is not failure, |metadata|["{{IdentityProviderClientMetadata/privacy_policy_url}}"]
                 is defined, and the |provider|'s {{IdentityProviderConfig/clientId}} is not in the list of
                 |account|["{{IdentityProviderAccount/approved_clients}}"], then the user agent MUST display