Mandate one presentation protocol MUST be supported

index.html

@@ -566,7 +566,24 @@ <h2 id="protocols">
       Use of the following [=digital credential/presentation protocols=] is
       defined by this specification.
     </p>
-    <aside class="issue" data-number="439"></aside>
+    <p>
+      A [=user agent=] MUST implement at least one of the [=digital
+      credential/presentation protocols=] &mdash; and it is RECOMMENDED
+      that they implement <em>all</em> [=digital credential/presentation
+      and issuance protocols=] &mdash; listed in the [=table of
+      supported presentation and issuance protocols=].
+    </p>
+    <aside class="note" title="Checking which protocols a user agent allows">
+      <p>
+        Developers can learn which
+        [=digital credential/presentation protocols=] or
+        [=digital credential/issuance protocols=] a user agent
+        allows by calling the
+        `DigitalCredential.`{{DigitalCredential.userAgentAllowsProtocol()}}
+        static method. See [[[#checking-if-protocol-is-allowed]]] for a usage
+        example.
+      </p>
+    </aside>
     <table class="data">
       <caption>
         <dfn class="export">Table of supported [=digital