diff --git a/index.bs b/index.bs
index 6fdd386f9..ef7cf6d82 100644
--- a/index.bs
+++ b/index.bs
@@ -119,6 +119,11 @@ spec: FIDO-CTAP; urlPrefix: https://fidoalliance.org/specs/fido-v2.0-ps-20170927
     type: dfn
         text: CTAP canonical CBOR encoding form; url: message-encoding
 
+spec: FIDO-APPID; urlPrefix: https://fidoalliance.org/specs/fido-u2f-v1.2-ps-20170411/fido-appid-and-facets-v1.2-ps-20170411.html
+    type: dfn
+        text: determining the FacetID of a calling application; url: determining-the-facetid-of-a-calling-application
+        text: determining if a caller's FacetID is authorized for an AppID; url: determining-if-a-caller-s-facetid-is-authorized-for-an-appid
+
 </pre> <!-- class=anchors -->
 
 <!-- L128 spec:webappsec-credential-management-1; type:dictionary; for:/; text:CredentialRequestOptions -->
@@ -303,6 +308,11 @@ below and in [[#index-defined-elsewhere]].
     the Web IDL standard adds support for {{Promise}}s, which are now the preferred mechanism for asynchronous
     interaction in all new web APIs.
 
+: FIDO AppID
+:: The algorithms for [=determining the FacetID of a calling application=] and
+    [=determining if a caller's FacetID is authorized for an AppID=] (used only in
+    the `appid` extension) are defined by [[!FIDO-APPID]].
+
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",  "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in [[!RFC2119]].
 
@@ -1162,7 +1172,8 @@ When this method is invoked, the user agent MUST execute the following algorithm
 
     1. Let |userPresence| be a Boolean value set to the inverse of |userVerification|.
 
-    1. If <code>|options|.{{PublicKeyCredentialRequestOptions/allowCredentials}}</code>
+    1. <span id="allowCredentialDescriptorListCreation"></span>
+        If <code>|options|.{{PublicKeyCredentialRequestOptions/allowCredentials}}</code>
         <dl class="switch">
             :   [=list/is not empty=]
             ::  1. Let |allowCredentialDescriptorList| be a new [=list=].
@@ -3530,33 +3541,45 @@ IANA "WebAuthn Extension Identifier" registry established by [[!WebAuthn-Registr
 These are recommended for implementation by user agents targeting broad interoperability.
 
 
-## FIDO AppId Extension (appid) ## {#sctn-appid-extension}
+## FIDO AppID Extension (appid) ## {#sctn-appid-extension}
 
-This [=authentication extension=] allows [=[RPS]=] that have previously registered a
-credential using the legacy FIDO JavaScript APIs to request an assertion.
-Specifically, this extension allows [=[RPS]=] to specify an |appId| [[FIDO-APPID]]
-to overwrite the otherwise computed |rpId|.  This extension is only valid if
-used during the {{CredentialsContainer/get()}} call; other usage will result in client
-error.
+This [=client extension=] allows [=[RPS]=] that have previously registered a
+credential using the legacy FIDO JavaScript APIs to request an [=assertion=]. The
+FIDO APIs use an alternative identifier for [=relying parties=] called an |AppID|
+[[FIDO-APPID]], and any credentials created using those APIs will be bound to
+that identifier. Without this extension they would need to be re-registered in
+order to be bound to an [=RP ID=].
+
+This extension does not allow FIDO-compatible credentials to be created. Thus
+credentials created with WebAuthn are not backwards compatible with the FIDO
+JavaScript APIs.
 
 : Extension identifier
 :: `appid`
 
 : Client extension input
-:: A single JSON string specifying a FIDO |appId|.
+:: A single JSON string specifying a FIDO |AppID|.
 
 : Client extension processing
-:: If {{PublicKeyCredentialRequestOptions/rpId}} is present, return a DOMException
-    whose name is "{{NotAllowedError}}", and terminate this algorithm ([[#discover-from-external-source]]).
-
-    Otherwise, replace the calculation of |rpId| in Step 6 of [[#discover-from-external-source]] with the
-    following procedure:  The client uses the value of |appid| to perform
-    the AppId validation procedure (as defined by [[FIDO-APPID]]).  If valid,
-    the value of |rpId| for all client processing should be replaced by the
-    value of |appid|.
+::  1. If present in a {{CredentialsContainer/create()}} call, return a
+        "{{NotSupportedError}}" {{DOMException}}—this extension is only valid when
+        requesting an assertion.
+    1. Let |facetId| be the result of passing the caller's [=origin=] to the
+        FIDO algorithm for [=determining the FacetID of a calling application=].
+    1. Let |appId| be the extension input.
+    1. Pass |facetId| and |appId| to the FIDO algorithm for [=determining if a
+        caller's FacetID is authorized for an AppID=]. If that algorithm rejects
+        |appId| then return a "{{SecurityError}}" {{DOMException}}.
+    1. When [building allowCredentialDescriptorList](#allowCredentialDescriptorListCreation),
+        if a U2F authenticator indicates that a credential is inapplicable (i.e. by
+        returning `SW_WRONG_DATA`) then the client MUST retry with the U2F application
+        parameter set to the SHA-256 hash of |appId|. If this results in an applicable
+        credential, the client MUST include the credential in
+        |allowCredentialDescriptorList|. The value of |appId| then replaces the `rpId`
+        parameter of [=authenticatorGetAssertion=].
 
 : Client extension output
-:: Returns the JSON value `true` to indicate to the RP that the extension was acted upon
+:: Returns the JSON value `true` to indicate to the RP that the extension was acted upon.
 
 : Authenticator extension input
 :: None.