diff --git a/dom/media/mediasource/SourceBuffer.cpp b/dom/media/mediasource/SourceBuffer.cpp
index 96cfc0d39f3b..71103bfd3111 100644
--- a/dom/media/mediasource/SourceBuffer.cpp
+++ b/dom/media/mediasource/SourceBuffer.cpp
@@ -301,6 +301,47 @@ void
 SourceBuffer::ChangeType(const nsAString& aType, ErrorResult& aRv)
 {
   MOZ_ASSERT(NS_IsMainThread());
+
+  DecoderDoctorDiagnostics diagnostics;
+  nsresult rv = MediaSource::IsTypeSupported(aType, &diagnostics);
+  diagnostics.StoreFormatDiagnostics(mMediaSource->GetOwner()
+                                     ? mMediaSource->GetOwner()->GetExtantDoc()
+                                     : nullptr,
+                                     aType, NS_SUCCEEDED(rv), __func__);
+  MSE_API("ChangeType(aType=%s)%s",
+          NS_ConvertUTF16toUTF8(aType).get(),
+          rv == NS_OK ? "" : " [not supported]");
+  if (NS_FAILED(rv)) {
+    DDLOG(DDLogCategory::API, "ChangeType", rv);
+    aRv.Throw(rv);
+    return;
+  }
+  if (!mMediaSource->GetDecoder() ||
+      mMediaSource->GetDecoder()->OwnerHasError()) {
+    MSE_DEBUG("HTMLMediaElement.error is not null");
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return;
+  }
+  if (!IsAttached() || mUpdating) {
+    DDLOG(DDLogCategory::API, "ChangeType", NS_ERROR_DOM_INVALID_STATE_ERR);
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return;
+  }
+
+  MOZ_ASSERT(mMediaSource->ReadyState() != MediaSourceReadyState::Closed);
+  if (mMediaSource->ReadyState() == MediaSourceReadyState::Ended) {
+    mMediaSource->SetReadyState(MediaSourceReadyState::Open);
+  }
+  if (mCurrentAttributes.GetAppendState() == AppendState::PARSING_MEDIA_SEGMENT){
+    DDLOG(DDLogCategory::API, "ChangeType", NS_ERROR_DOM_INVALID_STATE_ERR);
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return;
+  }
+  Maybe<MediaContainerType> containerType = MakeMediaContainerType(aType);
+  MOZ_ASSERT(containerType);
+  mType = *containerType;
+  ResetParserState();
+  mTrackBuffersManager->ChangeType(mType);
 }
 
 void
diff --git a/dom/media/mediasource/SourceBufferTask.h b/dom/media/mediasource/SourceBufferTask.h
index b73094fbd282..769ae3587a1d 100644
--- a/dom/media/mediasource/SourceBufferTask.h
+++ b/dom/media/mediasource/SourceBufferTask.h
@@ -24,7 +24,8 @@ public:
     Reset,
     RangeRemoval,
     EvictData,
-    Detach
+    Detach,
+    ChangeType
   };
 
   typedef Pair<bool, SourceBufferAttributes> AppendBufferResult;
@@ -112,6 +113,20 @@ public:
   const char* GetTypeName() const override { return "Detach"; }
 };
 
+class ChangeTypeTask : public SourceBufferTask {
+public:
+  explicit ChangeTypeTask(const MediaContainerType& aType)
+    : mType(aType)
+  {
+  }
+
+  static const Type sType = Type::ChangeType;
+  Type GetType() const override { return Type::ChangeType; }
+  const char* GetTypeName() const override { return "ChangeType"; }
+
+  const MediaContainerType mType;
+};
+
 } // end mozilla namespace
 
 #endif
diff --git a/dom/media/mediasource/TrackBuffersManager.cpp b/dom/media/mediasource/TrackBuffersManager.cpp
index ef2c5e1a2242..3de834a1e29a 100644
--- a/dom/media/mediasource/TrackBuffersManager.cpp
+++ b/dom/media/mediasource/TrackBuffersManager.cpp
@@ -111,6 +111,7 @@ TrackBuffersManager::TrackBuffersManager(MediaSourceDecoder* aParentDecoder,
   : mInputBuffer(new MediaByteBuffer)
   , mBufferFull(false)
   , mFirstInitializationSegmentReceived(false)
+  , mChangeTypeReceived(false)
   , mNewMediaSegmentStarted(false)
   , mActiveTrack(false)
   , mType(aType)
@@ -280,6 +281,14 @@ TrackBuffersManager::ProcessTasks()
       ShutdownDemuxers();
       ResetTaskQueue();
       return;
+    case Type::ChangeType:
+      MOZ_RELEASE_ASSERT(!mCurrentTask);
+      mType = task->As<ChangeTypeTask>()->mType;
+      mChangeTypeReceived = true;
+      mInitData = nullptr;
+      CompleteResetParserState();
+      CreateDemuxerforMIMEType();
+      break;
     default:
       NS_WARNING("Invalid Task");
   }
@@ -385,6 +394,15 @@ TrackBuffersManager::EvictData(const TimeUnit& aPlaybackTime, int64_t aSize)
   return result;
 }
 
+void
+TrackBuffersManager::ChangeType(const MediaContainerType& aType)
+{
+  MOZ_ASSERT(NS_IsMainThread());
+
+  QueueTask(new ChangeTypeTask(aType));
+}
+
+
 TimeIntervals
 TrackBuffersManager::Buffered() const
 {
@@ -475,10 +493,13 @@ TrackBuffersManager::CompleteResetParserState()
   }
 
   // We could be left with a demuxer in an unusable state. It needs to be
-  // recreated. We store in the InputBuffer an init segment which will be parsed
-  // during the next Segment Parser Loop and a new demuxer will be created and
-  // initialized.
-  if (mFirstInitializationSegmentReceived) {
+  // recreated. Unless we have a pending changeType operation, we store in the
+  // InputBuffer an init segment which will be parsed during the next Segment
+  // Parser Loop and a new demuxer will be created and initialized.
+  // If we are in the middle of a changeType operation, then we do not have an
+  // init segment yet. The next appendBuffer operation will need to provide such
+  // init segment.
+  if (mFirstInitializationSegmentReceived && !mChangeTypeReceived) {
     MOZ_ASSERT(mInitData && mInitData->Length(), "we must have an init segment");
     // The aim here is really to destroy our current demuxer.
     CreateDemuxerforMIMEType();
@@ -486,8 +507,10 @@ TrackBuffersManager::CompleteResetParserState()
     // to mInputBuffer as it will get modified in the Segment Parser Loop.
     mInputBuffer = new MediaByteBuffer;
     mInputBuffer->AppendElements(*mInitData);
+    RecreateParser(true);
+  } else {
+    RecreateParser(false);
   }
-  RecreateParser(true);
 }
 
 int64_t
@@ -721,7 +744,7 @@ TrackBuffersManager::SegmentParserLoop()
       MediaResult haveInitSegment = mParser->IsInitSegmentPresent(mInputBuffer);
       if (NS_SUCCEEDED(haveInitSegment)) {
         SetAppendState(AppendState::PARSING_INIT_SEGMENT);
-        if (mFirstInitializationSegmentReceived) {
+        if (mFirstInitializationSegmentReceived && !mChangeTypeReceived) {
           // This is a new initialization segment. Obsolete the old one.
           RecreateParser(false);
         }
@@ -772,8 +795,12 @@ TrackBuffersManager::SegmentParserLoop()
       return;
     }
     if (mSourceBufferAttributes->GetAppendState() == AppendState::PARSING_MEDIA_SEGMENT) {
-      // 1. If the first initialization segment received flag is false, then run the append error algorithm with the decode error parameter set to true and abort this algorithm.
-      if (!mFirstInitializationSegmentReceived) {
+      // 1. If the first initialization segment received flag is false, then run
+      //    the append error algorithm with the decode error parameter set to
+      //    true and abort this algorithm.
+      //    Or we are in the process of changeType, in which case we must first
+      //    get an init segment before getting a media segment.
+      if (!mFirstInitializationSegmentReceived || mChangeTypeReceived) {
         RejectAppend(NS_ERROR_FAILURE, __func__);
         return;
       }
@@ -1108,13 +1135,17 @@ TrackBuffersManager::OnDemuxerInitDone(const MediaResult& aResult)
   if (mFirstInitializationSegmentReceived) {
     if (numVideos != mVideoTracks.mNumTracks ||
         numAudios != mAudioTracks.mNumTracks ||
-        (numVideos && info.mVideo.mMimeType != mVideoTracks.mInfo->mMimeType) ||
-        (numAudios && info.mAudio.mMimeType != mAudioTracks.mInfo->mMimeType)) {
+        (!mChangeTypeReceived &&
+         ((numVideos &&
+           info.mVideo.mMimeType != mVideoTracks.mInfo->mMimeType) ||
+          (numAudios &&
+           info.mAudio.mMimeType != mAudioTracks.mInfo->mMimeType)))) {
       RejectAppend(NS_ERROR_FAILURE, __func__);
       return;
     }
-    // 1. If more than one track for a single type are present (ie 2 audio tracks),
-    // then the Track IDs match the ones in the first initialization segment.
+    // 1. If more than one track for a single type are present (ie 2 audio
+    // tracks), then the Track IDs match the ones in the first initialization
+    // segment.
     // TODO
     // 2. Add the appropriate track descriptions from this initialization
     // segment to each of the track buffers.
@@ -1214,6 +1245,9 @@ TrackBuffersManager::OnDemuxerInitDone(const MediaResult& aResult)
     mVideoTracks.mLastInfo = new TrackInfoSharedPtr(info.mVideo, streamID);
   }
 
+  // We have now completed the changeType operation.
+  mChangeTypeReceived = false;
+
   UniquePtr<EncryptionInfo> crypto = mInputDemuxer->GetCrypto();
   if (crypto && crypto->IsEncrypted()) {
     // Try and dispatch 'encrypted'. Won't go if ready state still HAVE_NOTHING.
diff --git a/dom/media/mediasource/TrackBuffersManager.h b/dom/media/mediasource/TrackBuffersManager.h
index 47fd1df8e326..9c7d191fc55c 100644
--- a/dom/media/mediasource/TrackBuffersManager.h
+++ b/dom/media/mediasource/TrackBuffersManager.h
@@ -115,6 +115,9 @@ public:
   // and if still more space is needed remove from the end.
   EvictDataResult EvictData(const media::TimeUnit& aPlaybackTime, int64_t aSize);
 
+  // Queue a task to run ChangeType
+  void ChangeType(const MediaContainerType& aType);
+
   // Returns the buffered range currently managed.
   // This may be called on any thread.
   // Buffered must conform to http://w3c.github.io/media-source/index.html#widl-SourceBuffer-buffered
@@ -205,10 +208,11 @@ private:
   // Accessed on both the main thread and the task queue.
   Atomic<bool> mBufferFull;
   bool mFirstInitializationSegmentReceived;
+  bool mChangeTypeReceived;
   // Set to true once a new segment is started.
   bool mNewMediaSegmentStarted;
   bool mActiveTrack;
-  const MediaContainerType mType;
+  MediaContainerType mType;
 
   // ContainerParser objects and methods.
   // Those are used to parse the incoming input buffer.