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diff --git a/dom/base/HighlightRegistry.h b/dom/base/HighlightRegistry.h
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+++ b/dom/base/HighlightRegistry.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_HighlightRegistry_h
+#define mozilla_dom_HighlightRegistry_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/CompactPair.h"
+#include "mozilla/dom/BindingDeclarations.h"
+#include "nsCycleCollectionParticipant.h"
+#include "nsHashKeys.h"
+#include "nsTHashMap.h"
+#include "nsHashtablesFwd.h"
+#include "nsWrapperCache.h"
+
+class nsFrameSelection;
+
+namespace mozilla {
+class ErrorResult;
+}
+namespace mozilla::dom {
+
+class AbstractRange;
+class Document;
+class Highlight;
+
+/**
+ * @brief HighlightRegistry manages all `Highlight`s available to a `Document`.
+ *
+ * This class is exposed via `HighlightRegistry.webidl` and used to
+ * add or remove `Highlight` instances to a document and binding it
+ * to a highlight name.
+ *
+ * The HighlightRegistry idl interface defines this class to be a `maplike`.
+ * To be able to access the members of the maplike without proper support
+ * for iteration from C++, the insertion and deletion operations are
+ * overridden and the data is also held inside of this class.
+ *
+ * @see https://drafts.csswg.org/css-highlight-api-1/#registration
+ */
+class HighlightRegistry final : public nsISupports, public nsWrapperCache {
+ public:
+  NS_DECL_CYCLE_COLLECTING_ISUPPORTS
+  NS_DECL_CYCLE_COLLECTION_WRAPPERCACHE_CLASS(HighlightRegistry)
+
+ public:
+  explicit HighlightRegistry(Document* aDocument);
+
+ protected:
+  ~HighlightRegistry();
+
+ public:
+  /**
+   * @brief Adds selections for all highlights to the `FrameSelection`.
+   *
+   * This method is called if highlights are added to the registry before
+   * a `FrameSelection` is available.
+   */
+  MOZ_CAN_RUN_SCRIPT void AddHighlightSelectionsToFrameSelection();
+
+  /**
+   * @brief Adds the Range to the Highlight Selection if it belongs to the same
+   * Document.
+   *
+   * If no Highlight Selection for this highlight exists, it will be created.
+   * This may occur when a Highlight is added to the Registry after the
+   * nsFrameSelection is created.
+   */
+  MOZ_CAN_RUN_SCRIPT void MaybeAddRangeToHighlightSelection(
+      AbstractRange& aRange, Highlight& aHighlight);
+
+  /**
+   * @brief Removes the Range from the Highlight Selection if it belongs to the
+   * same Document.
+   *
+   * @note If the last range of a highlight selection is removed, the selection
+   * itself is *not* removed.
+   */
+  MOZ_CAN_RUN_SCRIPT void MaybeRemoveRangeFromHighlightSelection(
+      AbstractRange& aRange, Highlight& aHighlight);
+
+  /**
+   * @brief Removes the highlight selections associated with the highlight.
+   *
+   * This method is called when the Highlight is cleared
+   * (i.e., all Ranges are removed).
+   */
+  MOZ_CAN_RUN_SCRIPT void RemoveHighlightSelection(Highlight& aHighlight);
+
+  // WebIDL interface
+
+  Document* GetParentObject() const { return mDocument; };
+
+  JSObject* WrapObject(JSContext* aCx,
+                       JS::Handle<JSObject*> aGivenProto) override;
+
+  /**
+   * @brief Adds a new `Highlight` to `this` using `aKey` as highlight name.
+   *
+   * Highlight instances are ordered by insertion.
+   *
+   * This call registers `this` and `aHighlightName` in the highlight given in
+   * `aValue`.
+   *
+   * If a `FrameSelection` is present, a highlight selection is created.
+   */
+  MOZ_CAN_RUN_SCRIPT void Set(const nsAString& aKey, Highlight& aValue,
+                              ErrorResult& aRv);
+
+  /**
+   * @brief Removes all highlights from this registry.
+   *
+   * If a `FrameSelection` is present, all highlight selections are removed.
+   */
+  MOZ_CAN_RUN_SCRIPT void Clear(ErrorResult& aRv);
+
+  /**
+   * @brief Removes the highlight named `aKey` from the registry.
+   *
+   * This call removes the combination of `this` and `aKey` from the highlight.
+   * If a `FrameSelection` is present, the highlight selection is removed.
+   *
+   * @return true if `aKey` existed and was deleted.
+   */
+  MOZ_CAN_RUN_SCRIPT bool Delete(const nsAString& aKey, ErrorResult& aRv);
+
+  /**
+   * @brief Get the `FrameSelection` object if available. Can return nullptr.
+   */
+  RefPtr<nsFrameSelection> GetFrameSelection();
+
+  /**
+   * @brief Get the registry name-value tuples.
+   */
+  nsTArray<CompactPair<RefPtr<nsAtom>, RefPtr<Highlight>>> const&
+  HighlightsOrdered() {
+    return mHighlightsOrdered;
+  }
+
+ private:
+  /**
+   * Parent document.
+   */
+  RefPtr<Document> mDocument;
+
+  /**
+   * Highlight instances are stored as array of name-value tuples
+   * instead of a hashmap in order to preserve the insertion order.
+   *
+   * This is done
+   *    a) to keep the order in sync with the underlying
+   *       data structure of the `maplike` interface and
+   *    b) because the insertion order defines the stacking order of
+   *       of highlights that have the same priority.
+   */
+  nsTArray<CompactPair<RefPtr<nsAtom>, RefPtr<Highlight>>> mHighlightsOrdered;
+};
+
+}  // namespace mozilla::dom
+
+#endif  // mozilla_dom_HighlightRegistry_h