diff --git a/content/media/mediasource/SourceBuffer.cpp b/content/media/mediasource/SourceBuffer.cpp
index 8889fdb1c561..f6575a16caf3 100644
--- a/content/media/mediasource/SourceBuffer.cpp
+++ b/content/media/mediasource/SourceBuffer.cpp
@@ -583,15 +583,9 @@ void
 SourceBuffer::AppendData(const uint8_t* aData, uint32_t aLength, ErrorResult& aRv)
 {
   MSE_DEBUG("SourceBuffer(%p)::AppendData(aLength=%u)", this, aLength);
-  if (!IsAttached() || mUpdating) {
-    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+  if (!PrepareAppend(aRv)) {
     return;
   }
-  if (mMediaSource->ReadyState() == MediaSourceReadyState::Ended) {
-    mMediaSource->SetReadyState(MediaSourceReadyState::Open);
-  }
-  // TODO: Run coded frame eviction algorithm.
-  // TODO: Test buffer full flag.
   StartUpdating();
   // TODO: Run more of the buffer append algorithm asynchronously.
   if (mParser->IsInitSegmentPresent(aData, aLength)) {
@@ -647,6 +641,28 @@ SourceBuffer::AppendData(const uint8_t* aData, uint32_t aLength, ErrorResult& aR
     return;
   }
 
+  // Schedule the state machine thread to ensure playback starts
+  // if required when data is appended.
+  mMediaSource->GetDecoder()->ScheduleStateMachineThread();
+
+  // Run the final step of the buffer append algorithm asynchronously to
+  // ensure the SourceBuffer's updating flag transition behaves as required
+  // by the spec.
+  nsCOMPtr<nsIRunnable> event = NS_NewRunnableMethod(this, &SourceBuffer::StopUpdating);
+  NS_DispatchToMainThread(event);
+}
+
+bool
+SourceBuffer::PrepareAppend(ErrorResult& aRv)
+{
+  if (!IsAttached() || mUpdating) {
+    aRv.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
+    return false;
+  }
+  if (mMediaSource->ReadyState() == MediaSourceReadyState::Ended) {
+    mMediaSource->SetReadyState(MediaSourceReadyState::Open);
+  }
+
   // Eviction uses a byte threshold. If the buffer is greater than the
   // number of bytes then data is evicted. The time range for this
   // eviction is reported back to the media source. It will then
@@ -665,15 +681,8 @@ SourceBuffer::AppendData(const uint8_t* aData, uint32_t aLength, ErrorResult& aR
     mMediaSource->NotifyEvicted(0.0, GetBufferedStart());
   }
 
-  // Run the final step of the buffer append algorithm asynchronously to
-  // ensure the SourceBuffer's updating flag transition behaves as required
-  // by the spec.
-  nsCOMPtr<nsIRunnable> event = NS_NewRunnableMethod(this, &SourceBuffer::StopUpdating);
-  NS_DispatchToMainThread(event);
-
-  // Schedule the state machine thread to ensure playback starts
-  // if required when data is appended.
-  mMediaSource->GetDecoder()->ScheduleStateMachineThread();
+  // TODO: Test buffer full flag.
+  return true;
 }
 
 double
diff --git a/content/media/mediasource/SourceBuffer.h b/content/media/mediasource/SourceBuffer.h
index 078422fd9a77..aa4e2f068b66 100644
--- a/content/media/mediasource/SourceBuffer.h
+++ b/content/media/mediasource/SourceBuffer.h
@@ -135,6 +135,10 @@ private:
   // Shared implementation of AppendBuffer overloads.
   void AppendData(const uint8_t* aData, uint32_t aLength, ErrorResult& aRv);
 
+  // Implements the "Prepare Append Algorithm".  Returns true if the append
+  // may continue, or false (with aRv set) on error.
+  bool PrepareAppend(ErrorResult& aRv);
+
   nsRefPtr<MediaSource> mMediaSource;
 
   const nsCString mType;