Clarify that extensions are optional

Fixes #74

index.src.html

@@ -1281,6 +1281,11 @@
 
 Extensions that are not supported are ignored.
 
+All WebAuthn extensions are optional for both clients and authenticators. Thus, any extensions requested by a [RP] may be
+ignored by the client browser or OS and not passed to the authenticator at all, or they may be ignored by the authenticator.
+Ignoring an extension is never considered a failure in the WebAuthn API, so when [RPS] include extensions with any API calls,
+they must be prepared to handle cases where some or all of those extensions are ignored.
+
 
 ## Extension identifiers ## {#extension-id}
 
@@ -1418,9 +1423,9 @@
 </pre>
 
 
-# Standard extensions # {#extension-standard}
+# Pre-defined extensions # {#extension-standard}
 
-This section defines standard extensions defined by the W3C.
+This section defines an initial set of extensions.
 
 
 ## Transaction authorization ## {#extension-txauth}