diff --git a/index.bs b/index.bs
index ccc9740..5f9e9bc 100644
--- a/index.bs
+++ b/index.bs
@@ -104,6 +104,7 @@ urlPrefix: https://www.w3.org/TR/appmanifest/; spec: appmanifest
         text: image object; url: #dfn-image-object
 urlPrefix: https://heycam.github.io/webidl/
     type: exception
+        text: DataError
         text: TypeError
 urlPrefix: https://tc39.github.io/ecma262/#sec-object.; type: dfn
     text: freeze
@@ -618,6 +619,69 @@ conforming IDL fragments, as described in the Web IDL specification. [[!WEBIDL]]
       </ol>
     </p>
   </section>
+
+  <section>
+    <h3 id='position-state'>Position State</h3>
+
+    <p>
+      A user agent MAY display the <a>current playback position</a> and <a>duration</a>
+      of a media session in the platform UI depending on platform conventions. The
+      <dfn>position state</dfn> is the combination of the following:
+      <ul>
+        <li>
+          The <dfn>duration</dfn> of the media.
+        </li>
+        <li>
+          The <dfn>playback rate</dfn> of the media.
+        </li>
+        <li>
+          The <dfn>last reported playback position</dfn> of the media. This is the
+          playback position of the media when the <a>position state</a> was created.
+        </li>
+      </ul>
+    </p>
+
+    <p>
+      The <a>position state</a> is represented by a {{MediaPositionState}} which MUST
+      always be stored with the <dfn>last position updated time</dfn>. This is the
+      time the <a>position state</a> was last updated.
+    </p>
+
+    <p>
+      The RECOMMENDED way to determine the <a>position state</a> is to monitor the
+      media elements whose node document's browsing context is the
+      <a>browsing context</a>.
+    </p>
+
+    <p>
+      The <dfn>current playback position</dfn> in seconds is computed in the following
+      way:
+      <ul>
+        <li>
+          The current time in seconds minus the <a>last position updated time</a> in
+          seconds.
+        </li>
+        <li>
+          Mutliply by the <a>playback rate</a>.
+        </li>
+        <li>
+          Add the <a>last reported playback position</a> in seconds.
+        </li>
+        <li>
+          If this value is less than zero, then return zero.
+        </li>
+        <li>
+          If this value is greater than the <a>duration</a> in seconds, then return the
+          <a>duration</a> in seconds.
+        </li>
+        <li>
+          Otherwise, return the value.
+        </li>
+      </ul>
+    </p>
+
+  </section>
+
 </section>
 
 <h2 id="the-mediasession-interface">The {{MediaSession}} interface</h2>
@@ -654,6 +718,8 @@ interface MediaSession {
   attribute MediaSessionPlaybackState playbackState;
 
   void setActionHandler(MediaSessionAction action, MediaSessionActionHandler? handler);
+
+  void setPositionState(MediaPositionState? state);
 };
 </pre>
 
@@ -748,6 +814,29 @@ interface MediaSession {
   <var>action</var> and <var>handler</var> on the {{MediaSession}}.
 </p>
 
+<p>
+  The <dfn method for=MediaSession>setPositionState()</dfn> method, when
+  invoked MUST perform the following steps:
+
+  <ul>
+    <li>
+      If the <var>state</var> is null then clear the <a>position state</a>.
+    </li>
+    <li>
+      If the <a dict-member for="MediaPositionState">duration</a> is negative,
+      throw a <a exception>DataError</a>.
+    </li>
+    <li>
+      If the <a dict-member for="MediaPositionState">position</a> is negative
+      or greater than <a dict-member for="MediaPositionState">duration</a>,
+      throw a <a exception>DataError</a>.
+    </li>
+    <li>
+      Update the <a>position state</a> and <a>last position updated time</a>.
+    </li>
+  </ul>
+</p>
+
 <h2 id="the-mediametadata-interface">The {{MediaMetadata}} interface</h2>
 
 <pre class="idl">
@@ -1016,6 +1105,34 @@ used to specify the {{MediaImage}} object's <a>MIME type</a>. It is a hint as to
 the media type of the image. The purpose of this attribute is to allow a user
 agent to ignore images of media types it does not support.
 
+<h2 id="the-mediapositionstate-dictionary">The {{MediaPositionState}} dictionary</h2>
+
+<pre class="idl">
+
+dictionary MediaPositionState {
+  required unrestricted double duration;
+  double playbackRate = 0.0;
+  double position = 0.0;
+};
+</pre>
+
+The {{MediaPositionState}} dictionary is a representation of the current playback
+position associated with a {{MediaSession}} that can be used by user agents to
+provide a user interface that displays the current playback position and duration.
+
+The <dfn dict-member for="MediaPositionState">duration</dfn> <a>dictionary member</a>
+is used to specify the <a>duration</a> in seconds. It should always be positive
+and positive infinity can be used to indicate media that is unending.
+
+The <dfn dict-member for="MediaPositionState">playbackRate</dfn> <a>dictionary member</a>
+is used to specify the <a>playback rate</a>. It can be positive to represent forward
+playback, negative to represent backwards playback or zero to represent media that
+is paused.
+
+The <dfn dict-member for="MediaPositionState">position</dfn> <a>dictionary member</a>
+is used to specify the <a>last reported playback position</a> in seconds. It should
+always be positive.
+
 <h2 id="examples">Examples</h2>
 
 <em>This section is non-normative.</em>
@@ -1186,6 +1303,33 @@ agent to ignore images of media types it does not support.
   </pre>
 </div>
 
+<div class="example" id="example-media-position-state">
+  Setting <a>position state</a>:
+  <pre class="lang-javascript">
+    // Media is loaded, set the duration.
+    navigator.mediaSession.setPositionState({
+      duration: 60
+    });
+
+    // Media starts playing at the beginning.
+    navigator.mediaSession.setPositionState({
+      duration: 60,
+      playbackRate: 1,
+      position: 0
+    });
+
+    // Media is paused 30 seconds in.
+    navigator.mediaSession.setPositionState({
+      duration: 60,
+      playbackRate: 0,
+      position: 30
+    });
+
+    // Media is reset.
+    navigator.mediaSession.setPositionState(null);
+  </pre>
+</div>
+
 <h2 id="acknowledgments" class="no-num">Acknowledgments</h2>
 
 The editors would like to thank Paul Adenot, Jake Archibald, Tab Atkins,