Relax setServerCertificate() requirement

encrypted-media-respec.html

@@ -3116,15 +3116,15 @@ <h2>
               Provides a server certificate to be used to encrypt messages to the license server.
             </p>
             <p>
-              [=Key Systems=] that use such certificates MUST also support requesting the
-              certificate from the server via the [=Queue a "message" Event=] algorithm.
+              For [=Key Systems=] that use such certificates, applications SHOULD call
+              {{MediaKeys/setServerCertificate()}} to explicitly set the server certificate.
+              Otherwise, license exchange may fail.
             </p>
             <p class="note">
-              This method allows an application to proactively provide a server certificate to
-              implementations that support it to avoid the additional round trip should the [=CDM=]
-              request it. It is intended as an optimization, and applications are not required to
-              use it.
-            </p>
+              Some key systems also support requesting the certificate from the server via the
+              [=Queue a "message" Event=] algorithm. However, this is not supported by all key
+              systems that use such certificates.
+            </p>          
             <p>
               The server certificate contents are [=Key System=]-specific. It MUST NOT contain
               executable code.