--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -3069,16 +3069,19 @@ HTMLMediaElement::ReportTelemetry()
           ("Reporting telemetry DROPPED_FRAMES_IN_VIDEO_PLAYBACK"));
       Telemetry::Accumulate(Telemetry::VIDEO_DROPPED_FRAMES_PROPORTION,
                             percentage);
     }
   }
 
   Telemetry::Accumulate(Telemetry::VIDEO_PLAY_TIME_MS, SECONDS_TO_MS(mPlayTime.Total()));
   LOG(LogLevel::Debug, ("%p VIDEO_PLAY_TIME_MS = %f", this, mPlayTime.Total()));
+
+  Telemetry::Accumulate(Telemetry::VIDEO_HIDDEN_PLAY_TIME_MS, SECONDS_TO_MS(mHiddenPlayTime.Total()));
+  LOG(LogLevel::Debug, ("%p VIDEO_HIDDEN_PLAY_TIME_MS = %f", this, mHiddenPlayTime.Total()));
 }
 
 void HTMLMediaElement::UnbindFromTree(bool aDeep,
                                       bool aNullParent)
 {
   if (!mPaused && mNetworkState != nsIDOMHTMLMediaElement::NETWORK_EMPTY) {
     Pause();
   }
@@ -4556,20 +4559,25 @@ nsresult HTMLMediaElement::DispatchAsync
     return NS_OK;
   }
 
   nsCOMPtr<nsIRunnable> event = new nsAsyncEventRunner(aName, this);
   NS_DispatchToMainThread(event);
 
   if ((aName.EqualsLiteral("play") || aName.EqualsLiteral("playing"))) {
     mPlayTime.Start();
+    if (IsHidden()) {
+      mHiddenPlayTime.Start();
+    }
   } else if (aName.EqualsLiteral("waiting")) {
     mPlayTime.Pause();
+    mHiddenPlayTime.Pause();
   } else if (aName.EqualsLiteral("pause")) {
     mPlayTime.Pause();
+    mHiddenPlayTime.Pause();
   }
 
   return NS_OK;
 }
 
 nsresult HTMLMediaElement::DispatchPendingMediaEvents()
 {
   NS_ASSERTION(!mEventDeliveryPaused,
@@ -4756,18 +4764,27 @@ void HTMLMediaElement::NotifyOwnerDocume
       NotifyAudioChannelAgent(false);
     }
   }
 }
 
 bool
 HTMLMediaElement::NotifyOwnerDocumentActivityChangedInternal()
 {
+  bool visible = !IsHidden();
+  if (visible) {
+    // Visible -> Just pause hidden play time (no-op if already paused).
+    mHiddenPlayTime.Pause();
+  } else if (mPlayTime.IsStarted()) {
+    // Not visible, play time is running -> Start hidden play time if needed.
+    mHiddenPlayTime.Start();
+  }
+
   if (mDecoder && !IsBeingDestroyed()) {
-    mDecoder->NotifyOwnerActivityChanged(!IsHidden());
+    mDecoder->NotifyOwnerActivityChanged(visible);
   }
 
   bool pauseElement = !IsActive();
   SuspendOrResumeElement(pauseElement, !IsActive());
 
   if (!mPausedForInactiveDocumentOrChannel &&
       mPlayBlockedBecauseHidden &&
       !OwnerDoc()->Hidden()) {

--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -1611,16 +1611,19 @@ public:
     TimeStamp mStartTime;
     TimeDuration mSum;
     uint32_t mCount;
   };
 private:
   // Total time a video has spent playing.
   TimeDurationAccumulator mPlayTime;
 
+  // Total time a video has spent playing while hidden.
+  TimeDurationAccumulator mHiddenPlayTime;
+
   // Indicates if user has interacted with the element.
   // Used to block autoplay when disabled.
   bool mHasUserInteraction;
 
   // True if the first frame has been successfully loaded.
   bool mFirstFrameLoaded;
 
   // Media elements also have a default playback start position, which must