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diff --git a/dom/base/Highlight.h b/dom/base/Highlight.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_Highlight_h
+#define mozilla_dom_Highlight_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "mozilla/dom/HighlightBinding.h"
+
+#include "nsCycleCollectionParticipant.h"
+#include "nsAtomHashKeys.h"
+#include "nsTHashSet.h"
+#include "nsTArray.h"
+#include "nsWrapperCache.h"
+
+class nsFrameSelection;
+class nsPIDOMWindowInner;
+namespace mozilla {
+class ErrorResult;
+}
+
+namespace mozilla::dom {
+class AbstractRange;
+class Document;
+class HighlightRegistry;
+class Selection;
+
+/**
+ * @brief Collection of all data of a highlight instance.
+ *
+ * This struct is intended to be allocated on the stack and passed on
+ * to the `nsFrameSelection` and layout code.
+ */
+struct HighlightSelectionData {
+  RefPtr<nsAtom> mHighlightName;
+  RefPtr<Highlight> mHighlight;
+};
+
+/**
+ * @brief Representation of a custom `Highlight`.
+ *
+ * A `Highlight` is defined in JS as a collection of `AbstractRange`s.
+ * Furthermore, a custom highlight contains the highlight type and priority.
+ *
+ * A highlight is added to a document using the `HighlightRegistry` interface.
+ * A highlight can be added to a document using different names as well as to
+ * multiple `HighlightRegistries`.
+ * To propagate runtime changes of the highlight to its registries, an
+ * observer pattern is implemented.
+ *
+ * The spec defines this class as a `setlike`. To allow access and iteration
+ * of the setlike contents from C++, the insertion and deletion operations are
+ * overridden and the Ranges are also stored internally in an Array.
+ *
+ * @see https://drafts.csswg.org/css-highlight-api-1/#creation
+ */
+class Highlight final : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(Highlight)
+
+ protected:
+  MOZ_CAN_RUN_SCRIPT Highlight(
+      const Sequence<OwningNonNull<AbstractRange>>& aInitialRanges,
+      nsPIDOMWindowInner* aWindow, ErrorResult& aRv);
+  ~Highlight() = default;
+
+ public:
+  /**
+   * @brief Adds `this` to `aHighlightRegistry`.
+   *
+   * Highlights must know of all registry objects which contain them, so that
+   * the registries can be notified when a property of the Highlight changes.
+   *
+   * Since a Highlight can be part of a registry using different names,
+   * the name has to be provided as well.
+   */
+  void AddToHighlightRegistry(HighlightRegistry& aHighlightRegistry,
+                              nsAtom& aHighlightName);
+
+  /**
+   * @brief Removes `this` from `aHighlightRegistry`.
+   */
+  void RemoveFromHighlightRegistry(HighlightRegistry& aHighlightRegistry,
+                                   nsAtom& aHighlightName);
+
+  /**
+   * @brief Creates a Highlight Selection using the given ranges.
+   */
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Selection> CreateHighlightSelection(
+      nsAtom* aHighlightName, nsFrameSelection* aFrameSelection);
+
+  // WebIDL interface
+  nsPIDOMWindowInner* GetParentObject() const { return mWindow; }
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  static MOZ_CAN_RUN_SCRIPT_BOUNDARY already_AddRefed<Highlight> Constructor(
+      const GlobalObject& aGlobal,
+      const Sequence<OwningNonNull<AbstractRange>>& aInitialRanges,
+      ErrorResult& aRv);
+
+  /**
+   * @brief Priority of this highlight.
+   *
+   * Priority is used to stack overlapping highlights.
+   */
+  int32_t Priority() const { return mPriority; }
+
+  /**
+   * @brief Set the priority of this Highlight.
+   *
+   * Priority is used to stack overlapping highlights.
+   */
+  void SetPriority(int32_t aPriority) { mPriority = aPriority; }
+
+  /**
+   * @brief The HighlightType of this Highlight (Highlight, Spelling Error,
+   * Grammar Error)
+   */
+  HighlightType Type() const { return mHighlightType; }
+
+  /**
+   * @brief Sets the HighlightType (Highlight, Spelling Error, Grammar Error)
+   */
+  void SetType(HighlightType aHighlightType) {
+    mHighlightType = aHighlightType;
+  }
+
+  /**
+   * @brief This mirrors the `size` property in JS world (_not_ exposed via
+   * webIDL)
+   */
+  uint32_t Size() const { return mRanges.Length(); }
+
+  /**
+   * @brief Adds a `Range` to this highlight.
+   *
+   * This adds `aRange` both to the setlike data storage and the internal one
+   * needed for iteration, if it is not yet present.
+   *
+   * Also notifies all `HighlightRegistry` instances.
+   */
+  MOZ_CAN_RUN_SCRIPT void Add(AbstractRange& aRange, ErrorResult& aRv);
+
+  /**
+   * @brief Removes all ranges from this highlight.
+   *
+   * This removes all highlights from the setlike data structure as well as from
+   * the internal one.
+   *
+   * Also notifies all `HighlightRegistry` instances.
+   */
+  MOZ_CAN_RUN_SCRIPT void Clear(ErrorResult& aRv);
+
+  /**
+   * @brief Removes `aRange` from this highlight.
+   *
+   * This removes `aRange` from the setlike data structure as well as from the
+   * internal one.
+   *
+   * Also notifies all `HighlightRegistry` instances.
+   *
+   * @return As per spec, returns true if the range was deleted.
+   */
+  MOZ_CAN_RUN_SCRIPT bool Delete(AbstractRange& aRange, ErrorResult& aRv);
+
+ private:
+  RefPtr<nsPIDOMWindowInner> mWindow;
+
+  /**
+   * All Range objects contained in this highlight.
+   */
+  nsTArray<RefPtr<AbstractRange>> mRanges;
+
+  /**
+   * Type of this highlight.
+   * @see HighlightType
+   */
+  HighlightType mHighlightType{HighlightType::Highlight};
+
+  /**
+   * Priority of this highlight.
+   *
+   * If highlights are overlapping, the priority can
+   * be used to prioritize. If the priorities of all
+   * Highlights involved are equal, the highlights are
+   * stacked in order of ther insertion into the
+   * `HighlightRegistry`.
+   */
+  int32_t mPriority{0};
+
+  /**
+   * All highlight registries that contain this Highlight.
+   *
+   * A highlight can be included in several registries
+   * using several names.
+   *
+   * Note: Storing `HighlightRegistry` as raw pointer is safe here
+   * because it unregisters itself from `this` when it is destroyed/CC'd
+   */
+  nsTHashMap<nsPtrHashKey<HighlightRegistry>, nsTHashSet<RefPtr<nsAtom>>>
+      mHighlightRegistries;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_Highlight_h