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diff --git a/dom/streams/ReadableStreamTee.h b/dom/streams/ReadableStreamTee.h
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+++ b/dom/streams/ReadableStreamTee.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_dom_ReadableStreamTee_h
+#define mozilla_dom_ReadableStreamTee_h
+
+#include "mozilla/dom/ReadRequest.h"
+#include "mozilla/dom/ReadableStreamController.h"
+#include "mozilla/dom/UnderlyingSourceCallbackHelpers.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsISupportsImpl.h"
+
+namespace mozilla::dom {
+struct TeeState;
+enum class TeeBranch : bool;
+class ReadableStream;
+
+// Implementation of the Pull algorithm steps for ReadableStreamDefaultTee,
+// from
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Step 12.
+class ReadableStreamDefaultTeeSourceAlgorithms final
+    : public UnderlyingSourceAlgorithmsBase {
+ public:
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(
+      ReadableStreamDefaultTeeSourceAlgorithms, UnderlyingSourceAlgorithmsBase)
+
+  ReadableStreamDefaultTeeSourceAlgorithms(TeeState* aTeeState,
+                                           TeeBranch aBranch)
+      : mTeeState(aTeeState), mBranch(aBranch) {}
+
+  MOZ_CAN_RUN_SCRIPT void StartCallback(JSContext* aCx,
+                                        ReadableStreamController& aController,
+                                        JS::MutableHandle<JS::Value> aRetVal,
+                                        ErrorResult& aRv) override {
+    aRetVal.setUndefined();
+  }
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> PullCallback(
+      JSContext* aCx, ReadableStreamController& aController,
+      ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Promise> CancelCallback(
+      JSContext* aCx, const Optional<JS::Handle<JS::Value>>& aReason,
+      ErrorResult& aRv) override;
+
+ protected:
+  ~ReadableStreamDefaultTeeSourceAlgorithms() override = default;
+
+ private:
+  // Virtually const, but is cycle collected
+  MOZ_KNOWN_LIVE RefPtr<TeeState> mTeeState;
+  TeeBranch mBranch;
+};
+
+// https://streams.spec.whatwg.org/#abstract-opdef-readablestreamdefaulttee
+// Step 12.3
+struct ReadableStreamDefaultTeeReadRequest final : public ReadRequest {
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(ReadableStreamDefaultTeeReadRequest,
+                                           ReadRequest)
+
+  RefPtr<TeeState> mTeeState;
+
+  explicit ReadableStreamDefaultTeeReadRequest(TeeState* aTeeState)
+      : mTeeState(aTeeState) {}
+
+  void ChunkSteps(JSContext* aCx, JS::Handle<JS::Value> aChunk,
+                  ErrorResult& aRv) override;
+
+  MOZ_CAN_RUN_SCRIPT void CloseSteps(JSContext* aCx, ErrorResult& aRv) override;
+
+  void ErrorSteps(JSContext* aCx, JS::Handle<JS::Value> aError,
+                  ErrorResult& aRv) override;
+
+ protected:
+  ~ReadableStreamDefaultTeeReadRequest() override = default;
+};
+
+namespace streams_abstract {
+MOZ_CAN_RUN_SCRIPT void ReadableByteStreamTee(
+    JSContext* aCx, ReadableStream* aStream,
+    nsTArray<RefPtr<ReadableStream>>& aResult, ErrorResult& aRv);
+}
+
+}  // namespace mozilla::dom
+#endif