diff --git a/dom/media/MediaStreamGraph.cpp b/dom/media/MediaStreamGraph.cpp
index 53144c054893..a23fba28f998 100644
--- a/dom/media/MediaStreamGraph.cpp
+++ b/dom/media/MediaStreamGraph.cpp
@@ -18,6 +18,7 @@
 #include "mozilla/Attributes.h"
 #include "TrackUnionStream.h"
 #include "ImageContainer.h"
+#include "AudioCaptureStream.h"
 #include "AudioChannelService.h"
 #include "AudioNodeEngine.h"
 #include "AudioNodeStream.h"
@@ -3192,6 +3193,17 @@ MediaStreamGraph::CreateTrackUnionStream(DOMMediaStream* aWrapper)
   return stream;
 }
 
+ProcessedMediaStream*
+MediaStreamGraph::CreateAudioCaptureStream(DOMMediaStream* aWrapper)
+{
+  AudioCaptureStream* stream = new AudioCaptureStream(aWrapper);
+  NS_ADDREF(stream);
+  MediaStreamGraphImpl* graph = static_cast<MediaStreamGraphImpl*>(this);
+  stream->SetGraphImpl(graph);
+  graph->AppendMessage(new CreateMessage(stream));
+  return stream;
+}
+
 AudioNodeExternalInputStream*
 MediaStreamGraph::CreateAudioNodeExternalInputStream(AudioNodeEngine* aEngine, TrackRate aSampleRate)
 {
@@ -3556,4 +3568,65 @@ ProcessedMediaStream::AddInput(MediaInputPort* aPort)
   GraphImpl()->SetStreamOrderDirty();
 }
 
+void
+MediaStreamGraph::RegisterCaptureStreamForWindow(
+    uint64_t aWindowId, ProcessedMediaStream* aCaptureStream)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MediaStreamGraphImpl* graphImpl = static_cast<MediaStreamGraphImpl*>(this);
+  graphImpl->RegisterCaptureStreamForWindow(aWindowId, aCaptureStream);
+}
+
+void
+MediaStreamGraphImpl::RegisterCaptureStreamForWindow(
+  uint64_t aWindowId, ProcessedMediaStream* aCaptureStream)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  WindowAndStream winAndStream;
+  winAndStream.mWindowId = aWindowId;
+  winAndStream.mCaptureStreamSink = aCaptureStream;
+  mWindowCaptureStreams.AppendElement(winAndStream);
+}
+
+void
+MediaStreamGraph::UnregisterCaptureStreamForWindow(uint64_t aWindowId)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  MediaStreamGraphImpl* graphImpl = static_cast<MediaStreamGraphImpl*>(this);
+  graphImpl->UnregisterCaptureStreamForWindow(aWindowId);
+}
+
+void
+MediaStreamGraphImpl::UnregisterCaptureStreamForWindow(uint64_t aWindowId)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  for (uint32_t i = 0; i < mWindowCaptureStreams.Length(); i++) {
+    if (mWindowCaptureStreams[i].mWindowId == aWindowId) {
+      mWindowCaptureStreams.RemoveElementAt(i);
+    }
+  }
+}
+
+already_AddRefed<MediaInputPort>
+MediaStreamGraph::ConnectToCaptureStream(uint64_t aWindowId,
+                                         MediaStream* aMediaStream)
+{
+  return aMediaStream->GraphImpl()->ConnectToCaptureStream(aWindowId,
+                                                           aMediaStream);
+}
+
+already_AddRefed<MediaInputPort>
+MediaStreamGraphImpl::ConnectToCaptureStream(uint64_t aWindowId,
+                                             MediaStream* aMediaStream)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+  for (uint32_t i = 0; i < mWindowCaptureStreams.Length(); i++) {
+    if (mWindowCaptureStreams[i].mWindowId == aWindowId) {
+      ProcessedMediaStream* sink = mWindowCaptureStreams[i].mCaptureStreamSink;
+      return sink->AllocateInputPort(aMediaStream, 0);
+    }
+  }
+  return nullptr;
+}
+
 } // namespace mozilla
diff --git a/dom/media/MediaStreamGraph.h b/dom/media/MediaStreamGraph.h
index 312aad706052..ac770fe1396a 100644
--- a/dom/media/MediaStreamGraph.h
+++ b/dom/media/MediaStreamGraph.h
@@ -1262,6 +1262,10 @@ public:
    * particular tracks of each input stream.
    */
   ProcessedMediaStream* CreateTrackUnionStream(DOMMediaStream* aWrapper);
+  /**
+   * Create a stream that will mix all its audio input.
+   */
+  ProcessedMediaStream* CreateAudioCaptureStream(DOMMediaStream* aWrapper);
   // Internal AudioNodeStreams can only pass their output to another
   // AudioNode, whereas external AudioNodeStreams can pass their output
   // to an nsAudioStream for playback.
@@ -1318,6 +1322,12 @@ public:
    */
   TrackRate GraphRate() const { return mSampleRate; }
 
+  void RegisterCaptureStreamForWindow(uint64_t aWindowId,
+                                      ProcessedMediaStream* aCaptureStream);
+  void UnregisterCaptureStreamForWindow(uint64_t aWindowId);
+  already_AddRefed<MediaInputPort> ConnectToCaptureStream(
+    uint64_t aWindowId, MediaStream* aMediaStream);
+
 protected:
   explicit MediaStreamGraph(TrackRate aSampleRate)
     : mNextGraphUpdateIndex(1)
diff --git a/dom/media/MediaStreamGraphImpl.h b/dom/media/MediaStreamGraphImpl.h
index 262cae526c2e..15ded10c470c 100644
--- a/dom/media/MediaStreamGraphImpl.h
+++ b/dom/media/MediaStreamGraphImpl.h
@@ -532,6 +532,13 @@ public:
     }
   }
 
+  // Capture Stream API. This allows to get a mixed-down output for a window.
+  void RegisterCaptureStreamForWindow(uint64_t aWindowId,
+                                      ProcessedMediaStream* aCaptureStream);
+  void UnregisterCaptureStreamForWindow(uint64_t aWindowId);
+  already_AddRefed<MediaInputPort>
+  ConnectToCaptureStream(uint64_t aWindowId, MediaStream* aMediaStream);
+
   // Data members
   //
   /**
@@ -755,6 +762,16 @@ private:
    * Used to pass memory report information across threads.
    */
   nsTArray<AudioNodeSizes> mAudioStreamSizes;
+
+  struct WindowAndStream
+  {
+    uint64_t mWindowId;
+    nsRefPtr<ProcessedMediaStream> mCaptureStreamSink;
+  };
+  /**
+   * Stream for window audio capture.
+   */
+  nsTArray<WindowAndStream> mWindowCaptureStreams;
   /**
    * Indicates that the MSG thread should gather data for a memory report.
    */