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diff --git a/layout/style/SharedStyleSheetCache.h b/layout/style/SharedStyleSheetCache.h
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+++ b/layout/style/SharedStyleSheetCache.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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_SharedStyleSheetCache_h__
+#define mozilla_SharedStyleSheetCache_h__
+
+// The shared style sheet cache is a cache that allows us to share sheets across
+// documents.
+//
+// It's generally a singleton, but it is different from GlobalStyleSheetCache in
+// the sense that:
+//
+//  * It needs to be cycle-collectable, as it can keep alive style sheets from
+//    various documents.
+//
+//  * It is conceptually a singleton, but given its cycle-collectable nature, we
+//    might re-create it.
+
+#include "mozilla/SharedSubResourceCache.h"
+#include "nsRefPtrHashtable.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/css/Loader.h"
+
+namespace mozilla {
+
+class StyleSheet;
+class SheetLoadDataHashKey;
+
+namespace css {
+class SheetLoadData;
+class Loader;
+}  // namespace css
+
+struct SharedStyleSheetCacheTraits {
+  using Loader = css::Loader;
+  using Key = SheetLoadDataHashKey;
+  using Value = StyleSheet;
+  using LoadingValue = css::SheetLoadData;
+
+  static SheetLoadDataHashKey KeyFromLoadingValue(const LoadingValue& aValue) {
+    return SheetLoadDataHashKey(aValue);
+  }
+};
+
+class SharedStyleSheetCache final
+    : public SharedSubResourceCache<SharedStyleSheetCacheTraits,
+                                    SharedStyleSheetCache>,
+      public nsIMemoryReporter {
+ public:
+  using Base = SharedSubResourceCache<SharedStyleSheetCacheTraits,
+                                      SharedStyleSheetCache>;
+
+  NS_DECL_ISUPPORTS
+  NS_DECL_NSIMEMORYREPORTER
+
+  SharedStyleSheetCache();
+  void Init();
+
+  // This has to be static because it's also called for loaders that don't have
+  // a sheet cache (loaders that are not owned by a document).
+  static void LoadCompleted(SharedStyleSheetCache*, css::SheetLoadData&,
+                            nsresult);
+  using Base::LoadCompleted;
+  static void LoadCompletedInternal(SharedStyleSheetCache*, css::SheetLoadData&,
+                                    nsTArray<RefPtr<css::SheetLoadData>>&);
+  static void Clear(nsIPrincipal* aForPrincipal = nullptr,
+                    const nsACString* aBaseDomain = nullptr);
+
+ protected:
+  void InsertIfNeeded(css::SheetLoadData&);
+
+  ~SharedStyleSheetCache();
+};
+
+}  // namespace mozilla
+
+#endif