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diff --git a/layout/style/PostTraversalTask.h b/layout/style/PostTraversalTask.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_PostTraversalTask_h
+#define mozilla_PostTraversalTask_h
+
+#include "nscore.h"
+
+/* a task to be performed immediately after a Servo traversal */
+
+namespace mozilla {
+class ServoStyleSet;
+namespace dom {
+class FontFace;
+class FontFaceSet;
+}  // namespace dom
+namespace fontlist {
+struct Family;
+}  // namespace fontlist
+}  // namespace mozilla
+class gfxUserFontEntry;
+
+namespace mozilla {
+
+/**
+ * A PostTraversalTask is a task to be performed immediately after a Servo
+ * traversal.  There are just a few tasks we need to perform, so we use this
+ * class rather than Runnables, to avoid virtual calls and some allocations.
+ *
+ * A PostTraversalTask is only safe to run immediately after the Servo
+ * traversal, since it can hold raw pointers to DOM objects.
+ */
+class PostTraversalTask {
+ public:
+  static PostTraversalTask ResolveFontFaceLoadedPromise(
+      dom::FontFace* aFontFace) {
+    auto task = PostTraversalTask(Type::ResolveFontFaceLoadedPromise);
+    task.mTarget = aFontFace;
+    return task;
+  }
+
+  static PostTraversalTask RejectFontFaceLoadedPromise(dom::FontFace* aFontFace,
+                                                       nsresult aResult) {
+    auto task = PostTraversalTask(Type::ResolveFontFaceLoadedPromise);
+    task.mTarget = aFontFace;
+    task.mResult = aResult;
+    return task;
+  }
+
+  static PostTraversalTask DispatchLoadingEventAndReplaceReadyPromise(
+      dom::FontFaceSet* aFontFaceSet) {
+    auto task =
+        PostTraversalTask(Type::DispatchLoadingEventAndReplaceReadyPromise);
+    task.mTarget = aFontFaceSet;
+    return task;
+  }
+
+  static PostTraversalTask DispatchFontFaceSetCheckLoadingFinishedAfterDelay(
+      dom::FontFaceSet* aFontFaceSet) {
+    auto task = PostTraversalTask(
+        Type::DispatchFontFaceSetCheckLoadingFinishedAfterDelay);
+    task.mTarget = aFontFaceSet;
+    return task;
+  }
+
+  static PostTraversalTask LoadFontEntry(gfxUserFontEntry* aFontEntry) {
+    auto task = PostTraversalTask(Type::LoadFontEntry);
+    task.mTarget = aFontEntry;
+    return task;
+  }
+
+  static PostTraversalTask InitializeFamily(fontlist::Family* aFamily) {
+    auto task = PostTraversalTask(Type::InitializeFamily);
+    task.mTarget = aFamily;
+    return task;
+  }
+
+  static PostTraversalTask FontInfoUpdate(ServoStyleSet* aSet) {
+    auto task = PostTraversalTask(Type::FontInfoUpdate);
+    task.mTarget = aSet;
+    return task;
+  }
+
+  void Run();
+
+ private:
+  // For any new raw pointer type that we need to store in a PostTraversalTask,
+  // please add an assertion that class' destructor that we are not in a Servo
+  // traversal, to protect against the possibility of having dangling pointers.
+  enum class Type {
+    // mTarget (FontFace*)
+    ResolveFontFaceLoadedPromise,
+
+    // mTarget (FontFace*)
+    // mResult
+    RejectFontFaceLoadedPromise,
+
+    // mTarget (FontFaceSet*)
+    DispatchLoadingEventAndReplaceReadyPromise,
+
+    // mTarget (FontFaceSet*)
+    DispatchFontFaceSetCheckLoadingFinishedAfterDelay,
+
+    // mTarget (gfxUserFontEntry*)
+    LoadFontEntry,
+
+    // mTarget (fontlist::Family*)
+    InitializeFamily,
+
+    // mTarget (ServoStyleSet*)
+    FontInfoUpdate,
+  };
+
+  explicit PostTraversalTask(Type aType)
+      : mType(aType), mTarget(nullptr), mResult(NS_OK) {}
+
+  Type mType;
+  void* mTarget;
+  nsresult mResult;
+};
+
+}  // namespace mozilla
+
+#endif  // mozilla_PostTraversalTask_h