Add voiceactivity action.

index.bs

@@ -412,6 +412,11 @@ platform UI or media keys, thereby improving the user experience.
           the action's intent is to open the media session in a
           picture-in-picture window.
         </li>
+        <li>
+          <dfn enum-value for=MediaSessionAction>voiceactivity</dfn>:
+          the action's intent is to notify the web page that a voice activity
+          has been detected by the microphone.
+        </li>
       </ul>
     </p>
 
@@ -541,6 +546,30 @@ platform UI or media keys, thereby improving the user experience.
       {{MediaSessionActionHandler}} before running, as different tasks, the
       steps defined to [$set a track's muted state$].
     </p>
+    <p>
+      A user agent MUST invoke the {{MediaSessionActionHandler}} for
+      {{MediaSessionAction/voiceactivity}} only when voice activity is detected
+      from a microphone with one or more live {{MediaStreamTrack}}s. A user
+      agent MAY ignore a {{MediaSessionAction/voiceactivity}} action if all
+      {{MediaStreamTrack}}s associated with the source are not
+      {{MediaStreamTrack/muted}}. It is RECOMMENDED for user agents to set a
+      minimal interval for invoking {{MediaSessionActionHandler}} for
+      {{MediaSessionAction/voiceactivity}} based on privacy and power efficiency
+      policies.
+    </p>
+
+    <p class=note>
+      {{MediaSessionAction/voiceactivity}} only indicates the start of a voice
+      activity. Application may display a notification if the user is speaking
+      while the {{MediaStreamTrack}} is muted, or start an {{AudioWorklet}} for
+      audio processing. No action is defined for the end of a voice activity.
+      Unlike other actions which are explicitely triggered by the user,
+      {{MediaSessionAction/voiceactivity}} also depends on the voice activity
+      detection algorithm of the user agent or the system. For privacy and power
+      efficiency concern, web page may not be notified if the second voice
+      activity started soon after last {{MediaSessionAction/voiceactivity}}
+      action.
+    </p>
 
     <p class=note>
       A page should only register a {{MediaSessionActionHandler}} for a <a>media
@@ -716,7 +745,8 @@ enum MediaSessionAction {
   "hangup",
   "previousslide",
   "nextslide",
-  "enterpictureinpicture"
+  "enterpictureinpicture",
+  "voiceactivity"
 };
 
 callback MediaSessionActionHandler = undefined(MediaSessionActionDetails details);
@@ -1496,6 +1526,7 @@ parameter whose dictionary type is:
   <li>{{MediaSessionActionDetails}} for {{MediaSessionAction/nextslide}}.</li>
   <li>{{MediaSessionActionDetails}} for
   {{MediaSessionAction/enterpictureinpicture}}.</li>
+  <li>{{MediaSessionActionDetails}} for {{MediaSessionAction/voiceactivity}}.</li>
 </ul>
 
 The <dfn dict-member for="MediaSessionActionDetails">action</dfn>
@@ -1807,6 +1838,21 @@ media session</a>.
   </pre>
 </div>
 
+<div class="example" id="example-enterpictureinpicture">
+  Handling voice activity:
+  <pre class="lang-javascript">
+    // Create a MediaStream with audio enabled.
+    const stream = await navigator.mediaDevices.getUserMedia({audio:true});
+    const track = stream.getAudioTracks()[0];
+    navigator.mediaSession.setActionHandler("voiceactivity", function() {
+      if (track.muted) {
+        // Show unmute notification. If user allows to unmute, call
+        // setMicrophoneActive(true) to unmute.
+      }
+    });
+  </pre>
+</div>
+
 <h2 id="acknowledgments" class="no-num">Acknowledgments</h2>
 
 The editors would like to thank Paul Adenot, Jake Archibald, Tab Atkins,