Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 4726 insertions, 0 deletions

diff --git a/editor/libeditor/HTMLEditor.h b/editor/libeditor/HTMLEditor.h
new file mode 100644
index 0000000000..2a31ead29d
--- /dev/null
+++ b/editor/libeditor/HTMLEditor.h
@@ -0,0 +1,4726 @@
+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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_HTMLEditor_h
+#define mozilla_HTMLEditor_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/ComposerCommandsUpdater.h"
+#include "mozilla/EditorBase.h"
+#include "mozilla/EditorForwards.h"
+#include "mozilla/ErrorResult.h"
+#include "mozilla/ManualNAC.h"
+#include "mozilla/Result.h"
+#include "mozilla/dom/BlobImpl.h"
+#include "mozilla/dom/Element.h"
+#include "mozilla/dom/File.h"
+
+#include "nsAttrName.h"
+#include "nsCOMPtr.h"
+#include "nsIDocumentObserver.h"
+#include "nsIDOMEventListener.h"
+#include "nsIEditorMailSupport.h"
+#include "nsIHTMLAbsPosEditor.h"
+#include "nsIHTMLEditor.h"
+#include "nsIHTMLInlineTableEditor.h"
+#include "nsIHTMLObjectResizer.h"
+#include "nsIPrincipal.h"
+#include "nsITableEditor.h"
+#include "nsPoint.h"
+#include "nsStubMutationObserver.h"
+
+#include <functional>
+
+class nsDocumentFragment;
+class nsFrameSelection;
+class nsHTMLDocument;
+class nsITransferable;
+class nsIClipboard;
+class nsRange;
+class nsStaticAtom;
+class nsStyledElement;
+class nsTableCellFrame;
+class nsTableWrapperFrame;
+template <class E>
+class nsTArray;
+
+namespace mozilla {
+class AlignStateAtSelection;
+class AutoSelectionSetterAfterTableEdit;
+class AutoSetTemporaryAncestorLimiter;
+class EmptyEditableFunctor;
+class ListElementSelectionState;
+class ListItemElementSelectionState;
+class ParagraphStateAtSelection;
+class ResizerSelectionListener;
+class Runnable;
+template <class T>
+class OwningNonNull;
+namespace dom {
+class AbstractRange;
+class Blob;
+class DocumentFragment;
+class Event;
+class HTMLBRElement;
+class MouseEvent;
+class StaticRange;
+}  // namespace dom
+namespace widget {
+struct IMEState;
+}  // namespace widget
+
+enum class ParagraphSeparator { div, p, br };
+
+/**
+ * The HTML editor implementation.<br>
+ * Use to edit HTML document represented as a DOM tree.
+ */
+class HTMLEditor final : public EditorBase,
+                         public nsIHTMLEditor,
+                         public nsIHTMLObjectResizer,
+                         public nsIHTMLAbsPosEditor,
+                         public nsITableEditor,
+                         public nsIHTMLInlineTableEditor,
+                         public nsStubMutationObserver,
+                         public nsIEditorMailSupport {
+ public:
+  /****************************************************************************
+   * NOTE: DO NOT MAKE YOUR NEW METHODS PUBLIC IF they are called by other
+   *       classes under libeditor except EditorEventListener and
+   *       HTMLEditorEventListener because each public method which may fire
+   *       eEditorInput event will need to instantiate new stack class for
+   *       managing input type value of eEditorInput and cache some objects
+   *       for smarter handling.  In other words, when you add new root
+   *       method to edit the DOM tree, you can make your new method public.
+   ****************************************************************************/
+
+  NS_DECL_ISUPPORTS_INHERITED
+  NS_DECL_CYCLE_COLLECTION_CLASS_INHERITED(HTMLEditor, EditorBase)
+
+  // nsStubMutationObserver overrides
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTAPPENDED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
+  NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
+  NS_DECL_NSIMUTATIONOBSERVER_CHARACTERDATACHANGED
+
+  // nsIHTMLEditor methods
+  NS_DECL_NSIHTMLEDITOR
+
+  // nsIHTMLObjectResizer methods (implemented in HTMLObjectResizer.cpp)
+  NS_DECL_NSIHTMLOBJECTRESIZER
+
+  // nsIHTMLAbsPosEditor methods (implemented in HTMLAbsPositionEditor.cpp)
+  NS_DECL_NSIHTMLABSPOSEDITOR
+
+  // nsIHTMLInlineTableEditor methods (implemented in HTMLInlineTableEditor.cpp)
+  NS_DECL_NSIHTMLINLINETABLEEDITOR
+
+  // nsIEditorMailSupport methods
+  NS_DECL_NSIEDITORMAILSUPPORT
+
+  // nsITableEditor methods
+  NS_DECL_NSITABLEEDITOR
+
+  // nsISelectionListener overrides
+  NS_DECL_NSISELECTIONLISTENER
+
+  /**
+   * @param aDocument   The document whose content will be editable.
+   */
+  explicit HTMLEditor(const Document& aDocument);
+
+  /**
+   * @param aDocument   The document whose content will be editable.
+   * @param aComposerCommandsUpdater     The composer command updater.
+   * @param aFlags      Some of nsIEditor::eEditor*Mask flags.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  Init(Document& aDocument, ComposerCommandsUpdater& aComposerCommandsUpdater,
+       uint32_t aFlags);
+
+  /**
+   * PostCreate() should be called after Init, and is the time that the editor
+   * tells its documentStateObservers that the document has been created.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult PostCreate();
+
+  /**
+   * PreDestroy() is called before the editor goes away, and gives the editor a
+   * chance to tell its documentStateObservers that the document is going away.
+   */
+  MOZ_CAN_RUN_SCRIPT void PreDestroy();
+
+  static HTMLEditor* GetFrom(nsIEditor* aEditor) {
+    return aEditor ? aEditor->GetAsHTMLEditor() : nullptr;
+  }
+  static const HTMLEditor* GetFrom(const nsIEditor* aEditor) {
+    return aEditor ? aEditor->GetAsHTMLEditor() : nullptr;
+  }
+
+  [[nodiscard]] bool GetReturnInParagraphCreatesNewParagraph() const;
+
+  // EditorBase overrides
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD BeginningOfDocument() final;
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD EndOfDocument() final;
+
+  NS_IMETHOD GetDocumentCharacterSet(nsACString& aCharacterSet) final;
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD
+  SetDocumentCharacterSet(const nsACString& aCharacterSet) final;
+
+  bool IsEmpty() const final;
+
+  bool CanPaste(int32_t aClipboardType) const final;
+  using EditorBase::CanPaste;
+
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD DeleteNode(nsINode* aNode,
+                                           bool aPreseveSelection,
+                                           uint8_t aOptionalArgCount) final;
+
+  MOZ_CAN_RUN_SCRIPT NS_IMETHOD InsertLineBreak() final;
+
+  /**
+   * PreHandleMouseDown() and PreHandleMouseUp() are called before
+   * HTMLEditorEventListener handles them.  The coming event may be
+   * non-acceptable event.
+   */
+  void PreHandleMouseDown(const dom::MouseEvent& aMouseDownEvent);
+  void PreHandleMouseUp(const dom::MouseEvent& aMouseUpEvent);
+
+  /**
+   * PreHandleSelectionChangeCommand() and PostHandleSelectionChangeCommand()
+   * are called before or after handling a command which may change selection
+   * and/or scroll position.
+   */
+  void PreHandleSelectionChangeCommand(Command aCommand);
+  void PostHandleSelectionChangeCommand(Command aCommand);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  HandleKeyPressEvent(WidgetKeyboardEvent* aKeyboardEvent) final;
+  Element* GetFocusedElement() const final;
+  bool IsActiveInDOMWindow() const final;
+  dom::EventTarget* GetDOMEventTarget() const final;
+  [[nodiscard]] Element* FindSelectionRoot(const nsINode& aNode) const final;
+  bool IsAcceptableInputEvent(WidgetGUIEvent* aGUIEvent) const final;
+  nsresult GetPreferredIMEState(widget::IMEState* aState) final;
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OnFocus(const nsINode& aOriginalEventTargetNode) final;
+  nsresult OnBlur(const dom::EventTarget* aEventTarget) final;
+
+  /**
+   * Called when aDocument or aElement becomes editable without focus change.
+   * E.g., when the design mode is enabled or the contenteditable attribute
+   * is set to the focused element.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult FocusedElementOrDocumentBecomesEditable(
+      Document& aDocument, Element* aElement);
+
+  /**
+   * Called when aDocument or aElement becomes not editable without focus
+   * change. E.g., when the design mode ends or the contenteditable attribute is
+   * removed or set to "false".
+   */
+  MOZ_CAN_RUN_SCRIPT static nsresult FocusedElementOrDocumentBecomesNotEditable(
+      HTMLEditor* aHTMLEditor, Document& aDocument, Element* aElement);
+
+  /**
+   * GetBackgroundColorState() returns what the background color of the
+   * selection.
+   *
+   * @param aMixed      true if there is more than one font color
+   * @param aOutColor   Color string. "" is returned for none.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult GetBackgroundColorState(bool* aMixed,
+                                                      nsAString& aOutColor);
+
+  /**
+   * PasteNoFormattingAsAction() pastes content in clipboard without any style
+   * information.
+   *
+   * @param aClipboardType      nsIClipboard::kGlobalClipboard or
+   *                            nsIClipboard::kSelectionClipboard.
+   * @param aDispatchPasteEvent Yes if this should dispatch ePaste event
+   *                            before pasting.  Otherwise, No.
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult PasteNoFormattingAsAction(
+      int32_t aClipboardType, DispatchPasteEvent aDispatchPasteEvent,
+      nsIPrincipal* aPrincipal = nullptr);
+
+  bool CanPasteTransferable(nsITransferable* aTransferable) final;
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  InsertLineBreakAsAction(nsIPrincipal* aPrincipal = nullptr) final;
+
+  /**
+   * InsertParagraphSeparatorAsAction() is called when user tries to separate
+   * current paragraph with Enter key press in HTMLEditor or something.
+   *
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  InsertParagraphSeparatorAsAction(nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  InsertElementAtSelectionAsAction(Element* aElement, bool aDeleteSelection,
+                                   nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult InsertLinkAroundSelectionAsAction(
+      Element* aAnchorElement, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * CreateElementWithDefaults() creates new element whose name is
+   * aTagName with some default attributes are set.  Note that this is a
+   * public utility method.  I.e., just creates element, not insert it
+   * into the DOM tree.
+   * NOTE: This is available for internal use too since this does not change
+   *       the DOM tree nor undo transactions, and does not refer Selection,
+   *       etc.
+   *
+   * @param aTagName            The new element's tag name.  If the name is
+   *                            one of "href", "anchor" or "namedanchor",
+   *                            this creates an <a> element.
+   * @return                    Newly created element.
+   */
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Element> CreateElementWithDefaults(
+      const nsAtom& aTagName);
+
+  /**
+   * Indent or outdent content around Selection.
+   *
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  IndentAsAction(nsIPrincipal* aPrincipal = nullptr);
+  MOZ_CAN_RUN_SCRIPT nsresult
+  OutdentAsAction(nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * The Document.execCommand("formatBlock") handler.
+   *
+   * @param aParagraphFormat    Must not be an empty string, and the value must
+   *                            be one of address, article, aside, blockquote,
+   *                            div, footer, h1, h2, h3, h4, h5, h6, header,
+   *                            hgroup, main, nav, p, pre, selection, dt or dd.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult FormatBlockAsAction(
+      const nsAString& aParagraphFormat, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * The cmd_paragraphState command handler.
+   *
+   * @param aParagraphFormat    Can be empty string.  If this is empty string,
+   *                            this removes ancestor format elements.
+   *                            Otherwise, the value must be one of p, pre,
+   *                            h1, h2, h3, h4, h5, h6, address, dt or dl.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult SetParagraphStateAsAction(
+      const nsAString& aParagraphFormat, nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult AlignAsAction(const nsAString& aAlignType,
+                                            nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult RemoveListAsAction(
+      const nsAString& aListType, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * MakeOrChangeListAsAction() makes selected hard lines list element(s).
+   *
+   * @param aListElementTagName         The new list element tag name.  Must be
+   *                                    nsGkAtoms::ul, nsGkAtoms::ol or
+   *                                    nsGkAtoms::dl.
+   * @param aBulletType                 If this is not empty string, it's set
+   *                                    to `type` attribute of new list item
+   *                                    elements.  Otherwise, existing `type`
+   *                                    attributes will be removed.
+   * @param aSelectAllOfCurrentList     Yes if this should treat all of
+   *                                    ancestor list element at selection.
+   */
+  enum class SelectAllOfCurrentList { Yes, No };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult MakeOrChangeListAsAction(
+      const nsStaticAtom& aListElementTagName, const nsAString& aBulletType,
+      SelectAllOfCurrentList aSelectAllOfCurrentList,
+      nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * If aTargetElement is a resizer, start to drag the resizer.  Otherwise, if
+   * aTargetElement is the grabber, start to handle drag gester on it.
+   *
+   * @param aMouseDownEvent     A `mousedown` event fired on aTargetElement.
+   * @param aEventTargetElement The target element being pressed.  This must
+   *                            be same as explicit original event target of
+   *                            aMouseDownEvent.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult StartToDragResizerOrHandleDragGestureOnGrabber(
+      dom::MouseEvent& aMouseDownEvent, Element& aEventTargetElement);
+
+  /**
+   * If the editor is handling dragging a resizer, handling drag gesture on
+   * the grabber or dragging the grabber, this finalize it.  Otherwise,
+   * does nothing.
+   *
+   * @param aClientPoint    The final point of the drag.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  StopDraggingResizerOrGrabberAt(const CSSIntPoint& aClientPoint);
+
+  /**
+   * If the editor is handling dragging a resizer, handling drag gesture to
+   * start dragging the grabber or dragging the grabber, this method updates
+   * it's position.
+   *
+   * @param aClientPoint    The new point of the drag.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  UpdateResizerOrGrabberPositionTo(const CSSIntPoint& aClientPoint);
+
+  /**
+   * IsCSSEnabled() returns true if this editor treats styles with style
+   * attribute of HTML elements.  Otherwise, if this editor treats all styles
+   * with "font style elements" like <b>, <i>, etc, and <blockquote> to indent,
+   * align attribute to align contents, returns false.
+   */
+  bool IsCSSEnabled() const { return mIsCSSPrefChecked; }
+
+  /**
+   * Enable/disable object resizers for <img> elements, <table> elements,
+   * absolute positioned elements (required absolute position editor enabled).
+   */
+  MOZ_CAN_RUN_SCRIPT void EnableObjectResizer(bool aEnable) {
+    if (mIsObjectResizingEnabled == aEnable) {
+      return;
+    }
+
+    AutoEditActionDataSetter editActionData(
+        *this, EditAction::eEnableOrDisableResizer);
+    if (NS_WARN_IF(!editActionData.CanHandle())) {
+      return;
+    }
+
+    mIsObjectResizingEnabled = aEnable;
+    RefreshEditingUI();
+  }
+  bool IsObjectResizerEnabled() const { return mIsObjectResizingEnabled; }
+
+  Element* GetResizerTarget() const { return mResizedObject; }
+
+  /**
+   * Enable/disable inline table editor, e.g., adding new row or column,
+   * removing existing row or column.
+   */
+  MOZ_CAN_RUN_SCRIPT void EnableInlineTableEditor(bool aEnable) {
+    if (mIsInlineTableEditingEnabled == aEnable) {
+      return;
+    }
+
+    AutoEditActionDataSetter editActionData(
+        *this, EditAction::eEnableOrDisableInlineTableEditingUI);
+    if (NS_WARN_IF(!editActionData.CanHandle())) {
+      return;
+    }
+
+    mIsInlineTableEditingEnabled = aEnable;
+    RefreshEditingUI();
+  }
+  bool IsInlineTableEditorEnabled() const {
+    return mIsInlineTableEditingEnabled;
+  }
+
+  /**
+   * Enable/disable absolute position editor, resizing absolute positioned
+   * elements (required object resizers enabled) or positioning them with
+   * dragging grabber.
+   */
+  MOZ_CAN_RUN_SCRIPT void EnableAbsolutePositionEditor(bool aEnable) {
+    if (mIsAbsolutelyPositioningEnabled == aEnable) {
+      return;
+    }
+
+    AutoEditActionDataSetter editActionData(
+        *this, EditAction::eEnableOrDisableAbsolutePositionEditor);
+    if (NS_WARN_IF(!editActionData.CanHandle())) {
+      return;
+    }
+
+    mIsAbsolutelyPositioningEnabled = aEnable;
+    RefreshEditingUI();
+  }
+  bool IsAbsolutePositionEditorEnabled() const {
+    return mIsAbsolutelyPositioningEnabled;
+  }
+
+  /**
+   * returns the deepest absolutely positioned container of the selection
+   * if it exists or null.
+   */
+  MOZ_CAN_RUN_SCRIPT already_AddRefed<Element>
+  GetAbsolutelyPositionedSelectionContainer() const;
+
+  Element* GetPositionedElement() const { return mAbsolutelyPositionedObject; }
+
+  /**
+   * extracts the selection from the normal flow of the document and
+   * positions it.
+   *
+   * @param aEnabled [IN] true to absolutely position the selection,
+   *                      false to put it back in the normal flow
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult SetSelectionToAbsoluteOrStaticAsAction(
+      bool aEnabled, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * returns the absolute z-index of a positioned element. Never returns 'auto'
+   * @return         the z-index of the element
+   * @param aElement [IN] the element.
+   */
+  MOZ_CAN_RUN_SCRIPT int32_t GetZIndex(Element& aElement);
+
+  /**
+   * adds aChange to the z-index of the currently positioned element.
+   *
+   * @param aChange [IN] relative change to apply to current z-index
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  AddZIndexAsAction(int32_t aChange, nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult SetBackgroundColorAsAction(
+      const nsAString& aColor, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * SetInlinePropertyAsAction() sets a property which changes inline style of
+   * text.  E.g., bold, italic, super and sub.
+   * This automatically removes exclusive style, however, treats all changes
+   * as a transaction.
+   *
+   * @param aPrincipal          Set subject principal if it may be called by
+   *                            JS.  If set to nullptr, will be treated as
+   *                            called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult SetInlinePropertyAsAction(
+      nsStaticAtom& aProperty, nsStaticAtom* aAttribute,
+      const nsAString& aValue, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * GetInlineProperty() gets aggregate properties of the current selection.
+   * All object in the current selection are scanned and their attributes are
+   * represented in a list of Property object.
+   * TODO: Make this return Result<Something> instead of bool out arguments.
+   *
+   * @param aHTMLProperty   the property to get on the selection
+   * @param aAttribute      the attribute of the property, if applicable.
+   *                        May be null.
+   *                        Example: aHTMLProperty=nsGkAtoms::font,
+   *                            aAttribute=nsGkAtoms::color
+   * @param aValue          if aAttribute is not null, the value of the
+   *                        attribute. May be null.
+   *                        Example: aHTMLProperty=nsGkAtoms::font,
+   *                            aAttribute=nsGkAtoms::color,
+   *                            aValue="0x00FFFF"
+   * @param aFirst          [OUT] true if the first text node in the
+   *                              selection has the property
+   * @param aAny            [OUT] true if any of the text nodes in the
+   *                              selection have the property
+   * @param aAll            [OUT] true if all of the text nodes in the
+   *                              selection have the property
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetInlineProperty(
+      nsStaticAtom& aHTMLProperty, nsAtom* aAttribute, const nsAString& aValue,
+      bool* aFirst, bool* aAny, bool* aAll) const;
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetInlinePropertyWithAttrValue(
+      nsStaticAtom& aHTMLProperty, nsAtom* aAttribute, const nsAString& aValue,
+      bool* aFirst, bool* aAny, bool* aAll, nsAString& outValue);
+
+  /**
+   * RemoveInlinePropertyAsAction() removes a property which changes inline
+   * style of text.  E.g., bold, italic, super and sub.
+   *
+   * @param aHTMLProperty   Tag name whcih represents the inline style you want
+   *                        to remove.  E.g., nsGkAtoms::strong, nsGkAtoms::b,
+   *                        etc.  If nsGkAtoms::href, <a> element which has
+   *                        href attribute will be removed.
+   *                        If nsGkAtoms::name, <a> element which has non-empty
+   *                        name attribute will be removed.
+   * @param aAttribute  If aHTMLProperty is nsGkAtoms::font, aAttribute should
+   *                    be nsGkAtoms::fase, nsGkAtoms::size, nsGkAtoms::color
+   *                    or nsGkAtoms::bgcolor.  Otherwise, set nullptr.
+   *                    Must not use nsGkAtoms::_empty here.
+   * @param aPrincipal  Set subject principal if it may be called by JS.  If
+   *                    set to nullptr, will be treated as called by system.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult RemoveInlinePropertyAsAction(
+      nsStaticAtom& aHTMLProperty, nsStaticAtom* aAttribute,
+      nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  RemoveAllInlinePropertiesAsAction(nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  IncreaseFontSizeAsAction(nsIPrincipal* aPrincipal = nullptr);
+
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DecreaseFontSizeAsAction(nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * GetFontColorState() returns foreground color information in first
+   * range of Selection.
+   * If first range of Selection is collapsed and there is a cache of style for
+   * new text, aIsMixed is set to false and aColor is set to the cached color.
+   * If first range of Selection is collapsed and there is no cached color,
+   * this returns the color of the node, aIsMixed is set to false and aColor is
+   * set to the color.
+   * If first range of Selection is not collapsed, this collects colors of
+   * each node in the range.  If there are two or more colors, aIsMixed is set
+   * to true and aColor is truncated.  If only one color is set to all of the
+   * range, aIsMixed is set to false and aColor is set to the color.
+   * If there is no Selection ranges, aIsMixed is set to false and aColor is
+   * truncated.
+   *
+   * @param aIsMixed            Must not be nullptr.  This is set to true
+   *                            if there is two or more colors in first
+   *                            range of Selection.
+   * @param aColor              Returns the color if only one color is set to
+   *                            all of first range in Selection.  Otherwise,
+   *                            returns empty string.
+   * @return                    Returns error only when illegal cases, e.g.,
+   *                            Selection instance has gone, first range
+   *                            Selection is broken.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  GetFontColorState(bool* aIsMixed, nsAString& aColor);
+
+  /**
+   * Detach aComposerCommandsUpdater from this.
+   */
+  void Detach(const ComposerCommandsUpdater& aComposerCommandsUpdater);
+
+  nsStaticAtom& DefaultParagraphSeparatorTagName() const {
+    return HTMLEditor::ToParagraphSeparatorTagName(mDefaultParagraphSeparator);
+  }
+  ParagraphSeparator GetDefaultParagraphSeparator() const {
+    return mDefaultParagraphSeparator;
+  }
+  void SetDefaultParagraphSeparator(ParagraphSeparator aSep) {
+    mDefaultParagraphSeparator = aSep;
+  }
+  static nsStaticAtom& ToParagraphSeparatorTagName(
+      ParagraphSeparator aSeparator) {
+    switch (aSeparator) {
+      case ParagraphSeparator::div:
+        return *nsGkAtoms::div;
+      case ParagraphSeparator::p:
+        return *nsGkAtoms::p;
+      case ParagraphSeparator::br:
+        return *nsGkAtoms::br;
+      default:
+        MOZ_ASSERT_UNREACHABLE("New paragraph separator isn't handled here");
+        return *nsGkAtoms::div;
+    }
+  }
+
+  /**
+   * Modifies the table containing the selection according to the
+   * activation of an inline table editing UI element
+   * @param aUIAnonymousElement [IN] the inline table editing UI element
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DoInlineTableEditingAction(const Element& aUIAnonymousElement);
+
+  /**
+   * GetInclusiveAncestorByTagName() looks for an element node whose name
+   * matches aTagName from aNode or anchor node of Selection to <body> element.
+   *
+   * @param aTagName        The tag name which you want to look for.
+   *                        Must not be nsGkAtoms::_empty.
+   *                        If nsGkAtoms::list, the result may be <ul>, <ol> or
+   *                        <dl> element.
+   *                        If nsGkAtoms::td, the result may be <td> or <th>.
+   *                        If nsGkAtoms::href, the result may be <a> element
+   *                        which has "href" attribute with non-empty value.
+   *                        If nsGkAtoms::anchor, the result may be <a> which
+   *                        has "name" attribute with non-empty value.
+   * @param aContent        Start node to look for the result.
+   * @return                If an element which matches aTagName, returns
+   *                        an Element.  Otherwise, nullptr.
+   */
+  Element* GetInclusiveAncestorByTagName(const nsStaticAtom& aTagName,
+                                         nsIContent& aContent) const;
+
+  /**
+   * Compute editing host for aContent.  If this editor isn't active in the DOM
+   * window, this returns nullptr.
+   */
+  enum class LimitInBodyElement { No, Yes };
+  [[nodiscard]] Element* ComputeEditingHost(
+      const nsIContent& aContent,
+      LimitInBodyElement aLimitInBodyElement = LimitInBodyElement::Yes) const {
+    return ComputeEditingHostInternal(&aContent, aLimitInBodyElement);
+  }
+
+  /**
+   * Compute editing host for the focus node of the Selection.  If this editor
+   * isn't active in the DOM window, this returns nullptr.
+   */
+  [[nodiscard]] Element* ComputeEditingHost(
+      LimitInBodyElement aLimitInBodyElement = LimitInBodyElement::Yes) const {
+    return ComputeEditingHostInternal(nullptr, aLimitInBodyElement);
+  }
+
+  /**
+   * Return true if we're in designMode.
+   */
+  bool IsInDesignMode() const;
+
+  /**
+   * Return true if entire the document is editable (although the document
+   * may have non-editable nodes, e.g.,
+   * <body contenteditable><div contenteditable="false"></div></body>
+   */
+  bool EntireDocumentIsEditable() const;
+
+  /**
+   * Basically, this always returns true if we're for `contenteditable` or
+   * `designMode` editor in web apps.  However, e.g., Composer of SeaMonkey
+   * can make the editor not tabbable.
+   */
+  bool IsTabbable() const { return IsInteractionAllowed(); }
+
+  /**
+   * NotifyEditingHostMaybeChanged() is called when new element becomes
+   * contenteditable when the document already had contenteditable elements.
+   */
+  void NotifyEditingHostMaybeChanged();
+
+  /** Insert a string as quoted text
+   * (whose representation is dependant on the editor type),
+   * replacing the selected text (if any).
+   *
+   * @param aQuotedText    The actual text to be quoted
+   * @parem aNodeInserted  Return the node which was inserted.
+   */
+  MOZ_CAN_RUN_SCRIPT  // USED_BY_COMM_CENTRAL
+      nsresult
+      InsertAsQuotation(const nsAString& aQuotedText, nsINode** aNodeInserted);
+
+  MOZ_CAN_RUN_SCRIPT nsresult InsertHTMLAsAction(
+      const nsAString& aInString, nsIPrincipal* aPrincipal = nullptr);
+
+  /**
+   * Refresh positions of resizers.  If you change size of target of resizers,
+   * you need to refresh position of resizers with calling this.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult RefreshResizers();
+
+  bool IsWrapHackEnabled() const {
+    return (mFlags & nsIEditor::eEditorEnableWrapHackMask) != 0;
+  }
+
+  /**
+   * Return true if this is in the plaintext mail composer mode of
+   * Thunderbird or something.
+   * NOTE: This is different from contenteditable="plaintext-only"
+   */
+  bool IsPlaintextMailComposer() const {
+    const bool isPlaintextMode =
+        (mFlags & nsIEditor::eEditorPlaintextMask) != 0;
+    MOZ_ASSERT_IF(IsTextEditor(), isPlaintextMode);
+    return isPlaintextMode;
+  }
+
+ protected:  // May be called by friends.
+  /****************************************************************************
+   * Some friend classes are allowed to call the following protected methods.
+   * However, those methods won't prepare caches of some objects which are
+   * necessary for them.  So, if you call them from friend classes, you need
+   * to make sure that AutoEditActionDataSetter is created.
+   ****************************************************************************/
+
+  /**
+   * InsertBRElement() creates a <br> element and inserts it before
+   * aPointToInsert.
+   *
+   * @param aWithTransaction    Whether the inserting is new element is undoable
+   *                            or not.  WithTransaction::No is useful only when
+   *                            the new element is inserted into a new element
+   *                            which has not been connected yet.
+   * @param aPointToInsert      The DOM point where should be <br> node inserted
+   *                            before.
+   * @param aSelect             If eNone, returns a point to put caret which is
+   *                            suggested by InsertNodeTransaction.
+   *                            If eNext, returns a point after the new <br>
+   *                            element.
+   *                            If ePrevious, returns a point at the new <br>
+   *                            element.
+   * @return                    The new <br> node and suggesting point to put
+   *                            caret which respects aSelect.
+   */
+  MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult> InsertBRElement(
+      WithTransaction aWithTransaction, const EditorDOMPoint& aPointToInsert,
+      EDirection aSelect = eNone);
+
+  /**
+   * Delete text in the range in aTextNode.  If aTextNode is not editable, this
+   * does nothing.
+   *
+   * @param aTextNode           The text node which should be modified.
+   * @param aOffset             Start offset of removing text in aTextNode.
+   * @param aLength             Length of removing text.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CaretPoint, nsresult>
+  DeleteTextWithTransaction(dom::Text& aTextNode, uint32_t aOffset,
+                            uint32_t aLength);
+
+  /**
+   * Replace text in the range with aStringToInsert.  If there is a DOM range
+   * exactly same as the replacing range, it'll be collapsed to
+   * {aTextNode, aOffset} because of the order of deletion and insertion.
+   * Therefore, the callers may need to handle `Selection` even when callers
+   * do not want to update `Selection`.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<InsertTextResult, nsresult>
+  ReplaceTextWithTransaction(dom::Text& aTextNode, uint32_t aOffset,
+                             uint32_t aLength,
+                             const nsAString& aStringToInsert);
+
+  /**
+   * Insert aStringToInsert to aPointToInsert.  If the point is not editable,
+   * this returns error.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<InsertTextResult, nsresult>
+  InsertTextWithTransaction(Document& aDocument,
+                            const nsAString& aStringToInsert,
+                            const EditorDOMPoint& aPointToInsert) final;
+
+  /**
+   * CopyLastEditableChildStyles() clones inline container elements into
+   * aPreviousBlock to aNewBlock to keep using same style in it.
+   *
+   * @param aPreviousBlock      The previous block element.  All inline
+   *                            elements which are last sibling of each level
+   *                            are cloned to aNewBlock.
+   * @param aNewBlock           New block container element.  All children of
+   *                            this is deleted first.
+   * @param aEditingHost        The editing host.
+   * @return                    If succeeded, returns a suggesting point to put
+   *                            caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  CopyLastEditableChildStylesWithTransaction(Element& aPreviousBlock,
+                                             Element& aNewBlock,
+                                             const Element& aEditingHost);
+
+  /**
+   * RemoveBlockContainerWithTransaction() removes aElement from the DOM tree
+   * but moves its all children to its parent node and if its parent needs <br>
+   * element to have at least one line-height, this inserts <br> element
+   * automatically.
+   *
+   * @param aElement            Block element to be removed.
+   * @return                    If succeeded, returns a suggesting point to put
+   *                            caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  RemoveBlockContainerWithTransaction(Element& aElement);
+
+  MOZ_CAN_RUN_SCRIPT nsresult RemoveAttributeOrEquivalent(
+      Element* aElement, nsAtom* aAttribute, bool aSuppressTransaction) final;
+  MOZ_CAN_RUN_SCRIPT nsresult SetAttributeOrEquivalent(
+      Element* aElement, nsAtom* aAttribute, const nsAString& aValue,
+      bool aSuppressTransaction) final;
+  using EditorBase::RemoveAttributeOrEquivalent;
+  using EditorBase::SetAttributeOrEquivalent;
+
+  /**
+   * Returns container element of ranges in Selection.  If Selection is
+   * collapsed, returns focus container node (or its parent element).
+   * If Selection selects only one element node, returns the element node.
+   * If Selection is only one range, returns common ancestor of the range.
+   * XXX If there are two or more Selection ranges, this returns parent node
+   *     of start container of a range which starts with different node from
+   *     start container of the first range.
+   */
+  Element* GetSelectionContainerElement() const;
+
+  /**
+   * DeleteTableCellContentsWithTransaction() removes any contents in cell
+   * elements.  If two or more cell elements are selected, this removes
+   * all selected cells' contents.  Otherwise, this removes contents of
+   * a cell which contains first selection range.  This does not return
+   * error even if selection is not in cell element, just does nothing.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult DeleteTableCellContentsWithTransaction();
+
+  /**
+   * extracts an element from the normal flow of the document and
+   * positions it, and puts it back in the normal flow.
+   * @param aElement [IN] the element
+   * @param aEnabled [IN] true to absolutely position the element,
+   *                      false to put it back in the normal flow
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult SetPositionToAbsoluteOrStatic(Element& aElement,
+                                                            bool aEnabled);
+
+  /**
+   * adds aChange to the z-index of an arbitrary element.
+   * @param aElement    [IN] the element
+   * @param aChange     [IN] relative change to apply to current z-index of
+   *                    the element
+   * @return            The new z-index of the element
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<int32_t, nsresult>
+  AddZIndexWithTransaction(nsStyledElement& aStyledElement, int32_t aChange);
+
+  /**
+   * Join together any adjacent editable text nodes in the range.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult CollapseAdjacentTextNodes(nsRange& aRange);
+
+  static dom::Element* GetLinkElement(nsINode* aNode);
+
+  /**
+   * Helper routines for font size changing.
+   */
+  enum class FontSize { incr, decr };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  SetFontSizeOnTextNode(Text& aTextNode, uint32_t aStartOffset,
+                        uint32_t aEndOffset, FontSize aIncrementOrDecrement);
+
+  enum class SplitAtEdges {
+    // SplitNodeDeepWithTransaction() won't split container element
+    // nodes at their edges.  I.e., when split point is start or end of
+    // container, it won't be split.
+    eDoNotCreateEmptyContainer,
+    // SplitNodeDeepWithTransaction() always splits containers even
+    // if the split point is at edge of a container.  E.g., if split point is
+    // start of an inline element, empty inline element is created as a new left
+    // node.
+    eAllowToCreateEmptyContainer,
+  };
+
+  /**
+   * SplitAncestorStyledInlineElementsAtRangeEdges() splits all ancestor inline
+   * elements in the block at aRange if given style matches with some of them.
+   *
+   * @param aRange              Ancestor inline elements of the start and end
+   *                            boundaries will be split.
+   * @param aStyle              The style which you want to split.
+   *                            RemoveAllStyles instance is allowed to split any
+   *                            inline elements.
+   * @param aSplitAtEdges       Whether this should split elements at start or
+   *                            end of inline elements or not.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitRangeOffResult, nsresult>
+  SplitAncestorStyledInlineElementsAtRangeEdges(const EditorDOMRange& aRange,
+                                                const EditorInlineStyle& aStyle,
+                                                SplitAtEdges aSplitAtEdges);
+
+  /**
+   * SplitAncestorStyledInlineElementsAt() splits ancestor inline elements at
+   * aPointToSplit if specified style matches with them.
+   *
+   * @param aPointToSplit       The point to split style at.
+   * @param aStyle              The style which you want to split.
+   *                            RemoveAllStyles instance is allowed to split any
+   *                            inline elements.
+   * @param aSplitAtEdges       Whether this should split elements at start or
+   *                            end of inline elements or not.
+   * @return                    The result of SplitNodeDeepWithTransaction()
+   *                            with topmost split element.  If this didn't
+   *                            find inline elements to be split, Handled()
+   *                            returns false.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  SplitAncestorStyledInlineElementsAt(const EditorDOMPoint& aPointToSplit,
+                                      const EditorInlineStyle& aStyle,
+                                      SplitAtEdges aSplitAtEdges);
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetInlinePropertyBase(
+      const EditorInlineStyle& aStyle, const nsAString* aValue, bool* aFirst,
+      bool* aAny, bool* aAll, nsAString* outValue) const;
+
+  /**
+   * ClearStyleAt() splits parent elements to remove the specified style.
+   * If this splits some parent elements at near their start or end, such
+   * empty elements will be removed.  Then, remove the specified style
+   * from the point and returns DOM point to put caret.
+   *
+   * @param aPoint      The point to clear style at.
+   * @param aStyleToRemove   The style which you want to clear.
+   * @param aSpecifiedStyle  Whether the class and style attributes should
+   *                         be preserved or discarded.
+   * @return            A candidate position to put caret.  If there is
+   *                    AutoTransactionsConserveSelection instances, this stops
+   *                    suggesting caret point only in some cases.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  ClearStyleAt(const EditorDOMPoint& aPoint,
+               const EditorInlineStyle& aStyleToRemove,
+               SpecifiedStyle aSpecifiedStyle);
+
+  MOZ_CAN_RUN_SCRIPT nsresult SetPositionToAbsolute(Element& aElement);
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetPositionToStatic(Element& aElement);
+
+  /**
+   * OnModifyDocument() is called when the editor is changed.  This should
+   * be called only by runnable in HTMLEditor::OnDocumentModified() to call
+   * HTMLEditor::OnModifyDocument() with AutoEditActionDataSetter instance.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult OnModifyDocument();
+
+  /**
+   * DoSplitNode() inserts aNewNode and moves all content before or after
+   * aStartOfRightNode to aNewNode.
+   *
+   * @param aStartOfRightNode   The point to split.  The container will keep
+   *                            having following or previous content of this.
+   * @param aNewNode            The new node called.  The previous or following
+   *                            content of aStartOfRightNode will be moved into
+   *                            this node.
+   */
+  MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult> DoSplitNode(
+      const EditorDOMPoint& aStartOfRightNode, nsIContent& aNewNode);
+
+  /**
+   * DoJoinNodes() merges contents in aContentToRemove to aContentToKeep and
+   * remove aContentToRemove from the DOM tree.  aContentToRemove and
+   * aContentToKeep must have same parent.  Additionally, if one of
+   * aContentToRemove or aContentToKeep is a text node, the other must be a
+   * text node.
+   *
+   * @param aContentToKeep    The node that will remain after the join.
+   * @param aContentToRemove  The node that will be joined with aContentToKeep.
+   *                          There is no requirement that the two nodes be of
+   *                          the same type.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  DoJoinNodes(nsIContent& aContentToKeep, nsIContent& aContentToRemove);
+
+  /**
+   * Routines for managing the preservation of selection across
+   * various editor actions.
+   */
+  bool ArePreservingSelection() const;
+  void PreserveSelectionAcrossActions();
+  MOZ_CAN_RUN_SCRIPT nsresult RestorePreservedSelection();
+  void StopPreservingSelection();
+
+  /**
+   * Called when JoinNodesTransaction::DoTransaction() did its transaction.
+   * Note that this is not called when undoing nor redoing.
+   *
+   * @param aTransaction        The transaction which did join nodes.
+   * @param aDoJoinNodesResult  Result of the doing join nodes.
+   */
+  MOZ_CAN_RUN_SCRIPT void DidJoinNodesTransaction(
+      const JoinNodesTransaction& aTransaction, nsresult aDoJoinNodesResult);
+
+ protected:  // edit sub-action handler
+  /**
+   * CanHandleHTMLEditSubAction() checks whether there is at least one
+   * selection range or not, and whether the first range is editable.
+   * If it's not editable, `Canceled()` of the result returns true.
+   * If `Selection` is in odd situation, returns an error.
+   *
+   * XXX I think that `IsSelectionEditable()` is better name, but it's already
+   *     in `EditorBase`...
+   */
+  enum class CheckSelectionInReplacedElement { Yes, OnlyWhenNotInSameNode };
+  Result<EditActionResult, nsresult> CanHandleHTMLEditSubAction(
+      CheckSelectionInReplacedElement aCheckSelectionInReplacedElement =
+          CheckSelectionInReplacedElement::Yes) const;
+
+  /**
+   * EnsureCaretNotAfterInvisibleBRElement() makes sure that caret is NOT after
+   * padding `<br>` element for preventing insertion after padding `<br>`
+   * element at empty last line.
+   * NOTE: This method should be called only when `Selection` is collapsed
+   *       because `Selection` is a pain to work with when not collapsed.
+   *       (no good way to extend start or end of selection), so we need to
+   *       ignore those types of selections.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  EnsureCaretNotAfterInvisibleBRElement();
+
+  /**
+   * MaybeCreatePaddingBRElementForEmptyEditor() creates padding <br> element
+   * for empty editor if there is no children.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  MaybeCreatePaddingBRElementForEmptyEditor();
+
+  /**
+   * EnsureNoPaddingBRElementForEmptyEditor() removes padding <br> element
+   * for empty editor if there is.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  EnsureNoPaddingBRElementForEmptyEditor();
+
+  /**
+   * ReflectPaddingBRElementForEmptyEditor() scans the tree from the root
+   * element and sets mPaddingBRElementForEmptyEditor if exists, or otherwise
+   * nullptr.  Can be used to manage undo/redo.
+   */
+  [[nodiscard]] nsresult ReflectPaddingBRElementForEmptyEditor();
+
+  /**
+   * PrepareInlineStylesForCaret() consider inline styles from top level edit
+   * sub-action and setting it to `mPendingStylesToApplyToNewContent` and clear
+   * inline style cache if necessary.
+   * NOTE: This method should be called only when `Selection` is collapsed.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult PrepareInlineStylesForCaret();
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  HandleInsertText(EditSubAction aEditSubAction,
+                   const nsAString& aInsertionString,
+                   SelectionHandling aSelectionHandling) final;
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertDroppedDataTransferAsAction(
+      AutoEditActionDataSetter& aEditActionData,
+      dom::DataTransfer& aDataTransfer, const EditorDOMPoint& aDroppedAt,
+      nsIPrincipal* aSourcePrincipal) final;
+
+  /**
+   * GetInlineStyles() retrieves the style of aElement and modifies each item of
+   * aPendingStyleCacheArray.  This might cause flushing layout at retrieving
+   * computed values of CSS properties.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetInlineStyles(
+      Element& aElement, AutoPendingStyleCacheArray& aPendingStyleCacheArray);
+
+  /**
+   * CacheInlineStyles() caches style of aElement into mCachedPendingStyles of
+   * TopLevelEditSubAction.  This may cause flushing layout at retrieving
+   * computed value of CSS properties.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  CacheInlineStyles(Element& aElement);
+
+  /**
+   * ReapplyCachedStyles() restores some styles which are disappeared during
+   * handling edit action and it should be restored.  This may cause flushing
+   * layout at retrieving computed value of CSS properties.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult ReapplyCachedStyles();
+
+  /**
+   * CreateStyleForInsertText() sets CSS properties which are stored in
+   * PendingStyles to proper element node.
+   *
+   * @param aPointToInsertText  The point to insert text.
+   * @param aEditingHost        The editing host.
+   * @return                    A suggest point to put caret or unset point.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  CreateStyleForInsertText(const EditorDOMPoint& aPointToInsertText,
+                           const Element& aEditingHost);
+
+  /**
+   * GetMostDistantAncestorMailCiteElement() returns most-ancestor mail cite
+   * element. "mail cite element" is <pre> element when it's in plaintext editor
+   * mode or an element with which calling HTMLEditUtils::IsMailCite() returns
+   * true.
+   *
+   * @param aNode       The start node to look for parent mail cite elements.
+   */
+  Element* GetMostDistantAncestorMailCiteElement(const nsINode& aNode) const;
+
+  /**
+   * HandleInsertParagraphInMailCiteElement() splits aMailCiteElement at
+   * aPointToSplit.
+   *
+   * @param aMailCiteElement    The mail-cite element which should be split.
+   * @param aPointToSplit       The point to split.
+   * @param aEditingHost        The editing host.
+   * @return                    Candidate caret position where is at inserted
+   *                            <br> element into the split point.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  HandleInsertParagraphInMailCiteElement(Element& aMailCiteElement,
+                                         const EditorDOMPoint& aPointToSplit,
+                                         const Element& aEditingHost);
+
+  /**
+   * HandleInsertBRElement() inserts a <br> element into aPointToBreak.
+   * This may split container elements at the point and/or may move following
+   * <br> element to immediately after the new <br> element if necessary.
+   *
+   * @param aPointToBreak       The point where new <br> element will be
+   *                            inserted before.
+   * @param aEditingHost        Current active editing host.
+   * @return                    If succeeded, returns new <br> element and
+   *                            candidate caret point.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  HandleInsertBRElement(const EditorDOMPoint& aPointToBreak,
+                        const Element& aEditingHost);
+
+  /**
+   * HandleInsertLinefeed() inserts a linefeed character into aInsertToBreak.
+   *
+   * @param aInsertToBreak      The point where new linefeed character will be
+   *                            inserted before.
+   * @param aEditingHost        Current active editing host.
+   * @return                    A suggest point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  HandleInsertLinefeed(const EditorDOMPoint& aInsertToBreak,
+                       const Element& aEditingHost);
+
+  /**
+   * Splits inclusive inline ancestors at both start and end of aRangeItem.  If
+   * this splits at every point, this modifies aRangeItem to point each split
+   * point (typically, at right node).
+   *
+   * @param aRangeItem          [in/out] One or two DOM points where should be
+   *                            split.  Will be modified to split point if
+   *                            they're split.
+   * @param aBlockInlineCheck   [in] Whether this method considers block vs.
+   *                            inline with computed style or the default style.
+   * @param aEditingHost        [in] The editing host.
+   * @param aAncestorLimiter    [in/optional] If specified, this stops splitting
+   *                            ancestors when meets this node.
+   * @return                    A suggest point to put caret if succeeded, but
+   *                            it may be unset.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  SplitInlineAncestorsAtRangeBoundaries(
+      RangeItem& aRangeItem, BlockInlineCheck aBlockInlineCheck,
+      const Element& aEditingHost,
+      const nsIContent* aAncestorLimiter = nullptr);
+
+  /**
+   * SplitElementsAtEveryBRElement() splits before all <br> elements in
+   * aMostAncestorToBeSplit.  All <br> nodes will be moved before right node
+   * at splitting its parent.  Finally, this returns left node, first <br>
+   * element, next left node, second <br> element... and right-most node.
+   *
+   * @param aMostAncestorToBeSplit      Most-ancestor element which should
+   *                                    be split.
+   * @param aOutArrayOfNodes            First left node, first <br> element,
+   *                                    Second left node, second <br> element,
+   *                                    ...right-most node.  So, all nodes
+   *                                    in this list should be siblings (may be
+   *                                    broken the relation by mutation event
+   *                                    listener though). If first <br> element
+   *                                    is first leaf node of
+   *                                    aMostAncestorToBeSplit, starting from
+   *                                    the first <br> element.
+   * @return                            A suggest point to put caret if
+   *                                    succeeded, but it may unset.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  SplitElementsAtEveryBRElement(
+      nsIContent& aMostAncestorToBeSplit,
+      nsTArray<OwningNonNull<nsIContent>>& aOutArrayOfContents);
+
+  /**
+   * MaybeSplitElementsAtEveryBRElement() calls SplitElementsAtEveryBRElement()
+   * for each given node when this needs to do that for aEditSubAction.
+   * If split a node, it in aArrayOfContents is replaced with split nodes and
+   * <br> elements.
+   *
+   * @return                            A suggest point to put caret if
+   *                                    succeeded, but it may unset.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  MaybeSplitElementsAtEveryBRElement(
+      nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      EditSubAction aEditSubAction);
+
+  /**
+   * CreateRangeIncludingAdjuscentWhiteSpaces() creates an nsRange instance
+   * which may be expanded from the given range to include adjuscent
+   * white-spaces.  If this fails handling something, returns nullptr.
+   */
+  template <typename EditorDOMRangeType>
+  already_AddRefed<nsRange> CreateRangeIncludingAdjuscentWhiteSpaces(
+      const EditorDOMRangeType& aRange);
+  template <typename EditorDOMPointType1, typename EditorDOMPointType2>
+  already_AddRefed<nsRange> CreateRangeIncludingAdjuscentWhiteSpaces(
+      const EditorDOMPointType1& aStartPoint,
+      const EditorDOMPointType2& aEndPoint);
+
+  /**
+   * GetRangeExtendedToHardLineEdgesForBlockEditAction() returns an extended
+   * range if aRange should be extended before handling a block level editing.
+   * If aRange start and/or end point <br> or something non-editable point, they
+   * should be moved to nearest text node or something where the other methods
+   * easier to handle edit action.
+   */
+  [[nodiscard]] Result<EditorRawDOMRange, nsresult>
+  GetRangeExtendedToHardLineEdgesForBlockEditAction(
+      const nsRange* aRange, const Element& aEditingHost) const;
+
+  /**
+   * InitializeInsertingElement is a callback type of methods which inserts
+   * an element into the DOM tree.  This is called immediately before inserting
+   * aNewElement into the DOM tree.
+   *
+   * @param aHTMLEditor     The HTML editor which modifies the DOM tree.
+   * @param aNewElement     The new element which will be or was inserted into
+   *                        the DOM tree.
+   * @param aPointToInsert  The position aNewElement will be or was inserted.
+   */
+  using InitializeInsertingElement =
+      std::function<nsresult(HTMLEditor& aHTMLEditor, Element& aNewElement,
+                             const EditorDOMPoint& aPointToInsert)>;
+  static InitializeInsertingElement DoNothingForNewElement;
+  static InitializeInsertingElement InsertNewBRElement;
+
+  /**
+   * Helper methods to implement InitializeInsertingElement.
+   */
+  MOZ_CAN_RUN_SCRIPT static Result<CreateElementResult, nsresult>
+  AppendNewElementToInsertingElement(
+      HTMLEditor& aHTMLEditor, const nsStaticAtom& aTagName,
+      Element& aNewElement,
+      const InitializeInsertingElement& aInitializer = DoNothingForNewElement);
+  MOZ_CAN_RUN_SCRIPT static Result<CreateElementResult, nsresult>
+  AppendNewElementWithBRToInsertingElement(HTMLEditor& aHTMLEditor,
+                                           const nsStaticAtom& aTagName,
+                                           Element& aNewElement);
+
+  /**
+   * Create an element node whose name is aTag at before aPointToInsert.  When
+   * this succeed to create an element node, this inserts the element to
+   * aPointToInsert.
+   *
+   * @param aWithTransaction    Whether the inserting is new element is undoable
+   *                            or not.  WithTransaction::No is useful only when
+   *                            the new element is inserted into a new element
+   *                            which has not been connected yet.
+   * @param aTagName            The element name to create.
+   * @param aPointToInsert      The insertion point of new element.
+   *                            If this refers end of the container or after,
+   *                            the transaction will append the element to the
+   *                            container.
+   *                            Otherwise, will insert the element before the
+   *                            child node referred by this.
+   *                            Note that this point will be invalid once this
+   *                            method inserts the new element.
+   * @param aInitializer        A function to initialize the new element before
+   *                            connecting the element into the DOM tree. Note
+   *                            that this should not touch outside given element
+   *                            because doing it would break range updater's
+   *                            result.
+   * @return                    The created new element node and candidate caret
+   *                            position.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  CreateAndInsertElement(
+      WithTransaction aWithTransaction, const nsAtom& aTagName,
+      const EditorDOMPoint& aPointToInsert,
+      const InitializeInsertingElement& aInitializer = DoNothingForNewElement);
+
+  /**
+   * Callback of CopyAttributes().
+   *
+   * @param aHTMLEditor   The HTML editor.
+   * @param aSrcElement   The element which have the attribute.
+   * @param aDestElement  The element which will have the attribute.
+   * @param aAttr         [in] The attribute which will be copied.
+   * @param aValue        [in/out] The attribute value which will be copied.
+   *                      Once updated, the new value is used.
+   * @return              true if the attribute should be copied, otherwise,
+   *                      false.
+   */
+  using AttributeFilter = std::function<bool(
+      HTMLEditor& aHTMLEditor, Element& aSrcElement, Element& aDestElement,
+      const dom::Attr& aAttr, nsString& aValue)>;
+  static AttributeFilter CopyAllAttributes;
+  static AttributeFilter CopyAllAttributesExceptId;
+  static AttributeFilter CopyAllAttributesExceptDir;
+  static AttributeFilter CopyAllAttributesExceptIdAndDir;
+
+  /**
+   * Copy all attributes of aSrcElement to aDestElement as-is.  Different from
+   * EditorBase::CloneAttributesWithTransaction(), this does not use
+   * SetAttributeOrEquivalent() nor does not clear existing attributes of
+   * aDestElement.
+   *
+   * @param aWithTransaction    Whether recoding with transactions or not.
+   * @param aDestElement        The element will have attributes.
+   * @param aSrcElement         The element whose attributes will be copied.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult CopyAttributes(
+      WithTransaction aWithTransaction, Element& aDestElement,
+      Element& aSrcElement, const AttributeFilter& = CopyAllAttributes);
+
+  /**
+   * MaybeSplitAncestorsForInsertWithTransaction() does nothing if container of
+   * aStartOfDeepestRightNode can have an element whose tag name is aTag.
+   * Otherwise, looks for an ancestor node which is or is in active editing
+   * host and can have an element whose name is aTag.  If there is such
+   * ancestor, its descendants are split.
+   *
+   * Note that this may create empty elements while splitting ancestors.
+   *
+   * @param aTag                        The name of element to be inserted
+   *                                    after calling this method.
+   * @param aStartOfDeepestRightNode    The start point of deepest right node.
+   *                                    This point must be in aEditingHost.
+   * @param aEditingHost                The editing host.
+   * @return                            When succeeded, SplitPoint() returns
+   *                                    the point to insert the element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  MaybeSplitAncestorsForInsertWithTransaction(
+      const nsAtom& aTag, const EditorDOMPoint& aStartOfDeepestRightNode,
+      const Element& aEditingHost);
+
+  /**
+   * InsertElementWithSplittingAncestorsWithTransaction() is a wrapper of
+   * MaybeSplitAncestorsForInsertWithTransaction() and CreateAndInsertElement().
+   * I.e., will create an element whose tag name is aTagName and split ancestors
+   * if it's necessary, then, insert it.
+   *
+   * @param aTagName            The tag name which you want to insert new
+   *                            element at aPointToInsert.
+   * @param aPointToInsert      The insertion point.  New element will be
+   *                            inserted before here.
+   * @param aBRElementNextToSplitPoint
+   *                            Whether <br> element should be deleted or
+   *                            kept if and only if a <br> element follows
+   *                            split point.
+   * @param aEditingHost        The editing host with which we're handling it.
+   * @param aInitializer        A function to initialize the new element before
+   *                            connecting the element into the DOM tree. Note
+   *                            that this should not touch outside given element
+   *                            because doing it would break range updater's
+   *                            result.
+   * @return                    If succeeded, returns the new element node and
+   *                            suggesting point to put caret.
+   */
+  enum class BRElementNextToSplitPoint { Keep, Delete };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  InsertElementWithSplittingAncestorsWithTransaction(
+      const nsAtom& aTagName, const EditorDOMPoint& aPointToInsert,
+      BRElementNextToSplitPoint aBRElementNextToSplitPoint,
+      const Element& aEditingHost,
+      const InitializeInsertingElement& aInitializer = DoNothingForNewElement);
+
+  /**
+   * Split aElementToSplit at two points, before aStartOfMiddleElement and after
+   * aEndOfMiddleElement.  If they are very start or very end of aBlockElement,
+   * this won't create empty block.
+   *
+   * @param aElementToSplit         An element which will be split.
+   * @param aStartOfMiddleElement   Start node of middle block element.
+   * @param aEndOfMiddleElement     End node of middle block element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitRangeOffFromNodeResult, nsresult>
+  SplitRangeOffFromElement(Element& aElementToSplit,
+                           nsIContent& aStartOfMiddleElement,
+                           nsIContent& aEndOfMiddleElement);
+
+  /**
+   * RemoveBlockContainerElementWithTransactionBetween() splits the nodes
+   * at aStartOfRange and aEndOfRange, then, removes the middle element which
+   * was split off from aBlockContainerElement and moves the ex-children to
+   * where the middle element was.  I.e., all nodes between aStartOfRange and
+   * aEndOfRange (including themselves) will be unwrapped from
+   * aBlockContainerElement.
+   *
+   * @param aBlockContainerElement  The node which will be split.
+   * @param aStartOfRange           The first node which will be unwrapped
+   *                                from aBlockContainerElement.
+   * @param aEndOfRange             The last node which will be unwrapped from
+   *                                aBlockContainerElement.
+   * @param aBlockInlineCheck       Whether this method considers block vs.
+   *                                inline with computed style or the default
+   *                                style.
+   * @return                        The left content is new created left
+   *                                element of aBlockContainerElement.
+   *                                The right content is split element,
+   *                                i.e., must be aBlockContainerElement.
+   *                                The middle content is nullptr since
+   *                                removing it is the job of this method.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitRangeOffFromNodeResult, nsresult>
+  RemoveBlockContainerElementWithTransactionBetween(
+      Element& aBlockContainerElement, nsIContent& aStartOfRange,
+      nsIContent& aEndOfRange, BlockInlineCheck aBlockInlineCheck);
+
+  /**
+   * WrapContentsInBlockquoteElementsWithTransaction() inserts at least one
+   * <blockquote> element and moves nodes in aArrayOfContents into new
+   * <blockquote> elements. If aArrayOfContents includes a table related element
+   * except <table>, this calls itself recursively to insert <blockquote> into
+   * the cell.
+   *
+   * @param aArrayOfContents    Nodes which will be moved into created
+   *                            <blockquote> elements.
+   * @param aEditingHost        The editing host.
+   * @return                    A blockquote element which is created at last
+   *                            and a suggest of caret position if succeeded.
+   *                            The caret suggestion may be unset if there is
+   *                            no suggestion.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  WrapContentsInBlockquoteElementsWithTransaction(
+      const nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      const Element& aEditingHost);
+
+  /**
+   * Our traditional formatBlock was same as XUL cmd_paragraphState command.
+   * However, the behavior is pretty different from the others and aligning
+   * the XUL command behavior may break Thunderbird a lot because it handles
+   * <blockquote> in a special path and <div> (generic block element) is not
+   * treated as a format node and these things may be used for designing
+   * current roles of the elements in the email composer of Thunderbird.
+   * Therefore, we create a new mode for HTMLFormatBlockCommand to align
+   * the behavior to the others but does not harm Thunderbird.
+   */
+  enum class FormatBlockMode {
+    // Document.execCommand("formatBlock"). Cannot set new format to "normal"
+    // nor "".  So, the paths to handle these ones are not used in this mode.
+    HTMLFormatBlockCommand,
+    // cmd_paragraphState.  Can set new format to "normal" or "" to remove
+    // ancestor format blocks.
+    XULParagraphStateCommand,
+  };
+
+  /**
+   * RemoveBlockContainerElementsWithTransaction() removes all format blocks,
+   * table related element, etc in aArrayOfContents from the DOM tree. If
+   * aArrayOfContents has a format node, it will be removed and its contents
+   * will be moved to where it was.
+   * If aArrayOfContents has a table related element, <li>, <blockquote> or
+   * <div>, it will be removed and its contents will be moved to where it was.
+   *
+   * @param aFormatBlockMode    Whether HTML formatBlock command or XUL
+   *                            paragraphState command.
+   *
+   * @return            A suggest point to put caret if succeeded, but it may be
+   *                    unset if there is no suggestion.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  RemoveBlockContainerElementsWithTransaction(
+      const nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      FormatBlockMode aFormatBlockMode, BlockInlineCheck aBlockInlineCheck);
+
+  /**
+   * CreateOrChangeFormatContainerElement() formats all nodes in
+   * aArrayOfContents with block elements whose name is aNewFormatTagName.
+   *
+   * If aArrayOfContents has an inline element, a block element is created and
+   * the inline element and following inline elements are moved into the new
+   * block element.
+   * If aArrayOfContents has <br> elements, they'll be removed from the DOM tree
+   * and new block element will be created when there are some remaining inline
+   * elements.
+   * If aArrayOfContents has a block element, this calls itself with children of
+   * the block element.  Then, new block element will be created when there are
+   * some remaining inline elements.
+   *
+   * @param aArrayOfContents    Must be descendants of a node.
+   * @param aNewFormatTagName   The element name of new block elements.
+   * @param aFormatBlockMode    The replacing block element target type is for
+   *                            whether HTML formatBLock command or XUL
+   *                            paragraphState command.
+   * @param aEditingHost        The editing host.
+   * @return                    The latest created new block element and a
+   *                            suggest point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  CreateOrChangeFormatContainerElement(
+      nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      const nsStaticAtom& aNewFormatTagName, FormatBlockMode aFormatBlockMode,
+      const Element& aEditingHost);
+
+  /**
+   * FormatBlockContainerWithTransaction() is implementation of "formatBlock"
+   * command of `Document.execCommand()`.  This applies block style or removes
+   * it.
+   *
+   * @param aSelectionRanges    The ranges which are cloned by selection or
+   *                            updated from it with doing something before
+   *                            calling this.
+   * @param aNewFormatTagName   New block tag name.
+   *                            If nsGkAtoms::normal or nsGkAtoms::_empty,
+   *                            RemoveBlockContainerElementsWithTransaction()
+   *                            will be called.
+   *                            If nsGkAtoms::blockquote,
+   *                            WrapContentsInBlockquoteElementsWithTransaction()
+   *                            will be called.
+   *                            Otherwise, CreateOrChangeBlockContainerElement()
+   *                            will be called.
+   * @param aFormatBlockMode    Whether HTML formatBlock command or XUL
+   *                            paragraphState command.
+   * @param aEditingHost        The editing host.
+   * @return                    If selection should be finally collapsed in a
+   *                            created block element, this returns the element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<RefPtr<Element>, nsresult>
+  FormatBlockContainerWithTransaction(AutoRangeArray& aSelectionRanges,
+                                      const nsStaticAtom& aNewFormatTagName,
+                                      FormatBlockMode aFormatBlockMode,
+                                      const Element& aEditingHost);
+
+  /**
+   * Determine if aPointToInsert is start of a hard line and end of the line
+   * (i.e, in an empty line) and the line ends with block boundary, inserts a
+   * `<br>` element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CaretPoint, nsresult>
+  InsertBRElementIfHardLineIsEmptyAndEndsWithBlockBoundary(
+      const EditorDOMPoint& aPointToInsert);
+
+  /**
+   * Insert padding `<br>` element for empty last line into aElement if
+   * aElement is a block element and empty.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  InsertPaddingBRElementForEmptyLastLineIfNeeded(Element& aElement);
+
+  /**
+   * This method inserts a padding `<br>` element for empty last line if
+   * selection is collapsed and container of the range needs it.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  MaybeInsertPaddingBRElementForEmptyLastLineAtSelection();
+
+  /**
+   * SplitParagraphWithTransaction() splits the parent block, aParentDivOrP, at
+   * aStartOfRightNode.
+   *
+   * @param aParentDivOrP       The parent block to be split.  This must be <p>
+   *                            or <div> element.
+   * @param aStartOfRightNode   The point to be start of right node after
+   *                            split.  This must be descendant of
+   *                            aParentDivOrP.
+   * @param aMayBecomeVisibleBRElement
+   *                            Next <br> element of the split point if there
+   *                            is.  Otherwise, nullptr. If this is not nullptr,
+   *                            the <br> element may be removed if it becomes
+   *                            visible.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  SplitParagraphWithTransaction(Element& aParentDivOrP,
+                                const EditorDOMPoint& aStartOfRightNode,
+                                dom::HTMLBRElement* aMayBecomeVisibleBRElement);
+
+  /**
+   * HandleInsertParagraphInParagraph() does the right thing for Enter key
+   * press or 'insertParagraph' command in aParentDivOrP.  aParentDivOrP will
+   * be split **around** aCandidatePointToSplit.  If this thinks that it should
+   * be handled to insert a <br> instead, this returns "not handled".
+   *
+   * @param aParentDivOrP   The parent block.  This must be <p> or <div>
+   *                        element.
+   * @param aCandidatePointToSplit
+   *                        The point where the caller want to split
+   *                        aParentDivOrP.  However, in some cases, this is not
+   *                        used as-is.  E.g., this method avoids to create new
+   *                        empty <a href> in the right paragraph.  So this may
+   *                        be adjusted to proper position around it.
+   * @param aEditingHost    The editing host.
+   * @return                If the caller should default to inserting <br>
+   *                        element, returns "not handled".
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  HandleInsertParagraphInParagraph(Element& aParentDivOrP,
+                                   const EditorDOMPoint& aCandidatePointToSplit,
+                                   const Element& aEditingHost);
+
+  /**
+   * HandleInsertParagraphInHeadingElement() handles insertParagraph command
+   * (i.e., handling Enter key press) in a heading element.  This splits
+   * aHeadingElement element at aPointToSplit.  Then, if right heading element
+   * is empty, it'll be removed and new paragraph is created (its type is
+   * decided with default paragraph separator).
+   *
+   * @param aHeadingElement     The heading element to be split.
+   * @param aPointToSplit       The point to split aHeadingElement.
+   * @return                    New paragraph element, meaning right heading
+   *                            element if aHeadingElement is split, or newly
+   *                            created or existing paragraph element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<InsertParagraphResult, nsresult>
+  HandleInsertParagraphInHeadingElement(Element& aHeadingElement,
+                                        const EditorDOMPoint& aPointToSplit);
+
+  /**
+   * HandleInsertParagraphInListItemElement() handles insertParagraph command
+   * (i.e., handling Enter key press) in a list item element.
+   *
+   * @param aListItemElement    The list item which has the following point.
+   * @param aPointToSplit       The point to split aListItemElement.
+   * @param aEditingHost        The editing host.
+   * @return                    New paragraph element, meaning right list item
+   *                            element if aListItemElement is split, or newly
+   *                            created paragraph element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<InsertParagraphResult, nsresult>
+  HandleInsertParagraphInListItemElement(Element& aListItemElement,
+                                         const EditorDOMPoint& aPointToSplit,
+                                         const Element& aEditingHost);
+
+  /**
+   * InsertParagraphSeparatorAsSubAction() handles insertPargraph commad
+   * (i.e., handling Enter key press) with the above helper methods.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  InsertParagraphSeparatorAsSubAction(const Element& aEditingHost);
+
+  /**
+   * InsertLineBreakAsSubAction() inserts a new <br> element or a linefeed
+   * character at selection.  If there is non-collapsed selection ranges, the
+   * selected ranges is deleted first.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertLineBreakAsSubAction();
+
+  /**
+   * ChangeListElementType() replaces child list items of aListElement with
+   * new list item element whose tag name is aNewListItemTag.
+   * Note that if there are other list elements as children of aListElement,
+   * this calls itself recursively even though it's invalid structure.
+   *
+   * @param aListElement        The list element whose list items will be
+   *                            replaced.
+   * @param aNewListTag         New list tag name.
+   * @param aNewListItemTag     New list item tag name.
+   * @return                    New list element or an error code if it fails.
+   *                            New list element may be aListElement if its
+   *                            tag name is same as aNewListTag.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  ChangeListElementType(Element& aListElement, nsAtom& aListType,
+                        nsAtom& aItemType);
+
+  class AutoListElementCreator;
+
+  [[nodiscard]] static bool IsFormatElement(FormatBlockMode aFormatBlockMode,
+                                            const nsIContent& aContent);
+
+  /**
+   * MakeOrChangeListAndListItemAsSubAction() handles create list commands with
+   * current selection.  If
+   *
+   * @param aListElementOrListItemElementTagName
+   *                                    The new list element tag name or
+   *                                    new list item tag name.
+   *                                    If the former, list item tag name will
+   *                                    be computed automatically.  Otherwise,
+   *                                    list tag name will be computed.
+   * @param aBulletType                 If this is not empty string, it's set
+   *                                    to `type` attribute of new list item
+   *                                    elements.  Otherwise, existing `type`
+   *                                    attributes will be removed.
+   * @param aSelectAllOfCurrentList     Yes if this should treat all of
+   *                                    ancestor list element at selection.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  MakeOrChangeListAndListItemAsSubAction(
+      const nsStaticAtom& aListElementOrListItemElementTagName,
+      const nsAString& aBulletType,
+      SelectAllOfCurrentList aSelectAllOfCurrentList);
+
+  /**
+   * DeleteTextAndTextNodesWithTransaction() removes text or text nodes in
+   * the given range.
+   */
+  enum class TreatEmptyTextNodes {
+    // KeepIfContainerOfRangeBoundaries:
+    //   Will remove empty text nodes middle of the range, but keep empty
+    //   text nodes which are containers of range boundaries.
+    KeepIfContainerOfRangeBoundaries,
+    // Remove:
+    //   Will remove all empty text nodes.
+    Remove,
+    // RemoveAllEmptyInlineAncestors:
+    //   Will remove all empty text nodes and its inline ancestors which
+    //   become empty due to removing empty text nodes.
+    RemoveAllEmptyInlineAncestors,
+  };
+  template <typename EditorDOMPointType>
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CaretPoint, nsresult>
+  DeleteTextAndTextNodesWithTransaction(
+      const EditorDOMPointType& aStartPoint,
+      const EditorDOMPointType& aEndPoint,
+      TreatEmptyTextNodes aTreatEmptyTextNodes);
+
+  /**
+   * JoinNodesWithTransaction() joins aLeftContent and aRightContent.  Content
+   * of aLeftContent will be merged into aRightContent.  Actual implemenation of
+   * this method is JoinNodesImpl().  So, see its explanation for the detail.
+   *
+   * @param aLeftContent   Will be removed from the DOM tree.
+   * @param aRightContent  The node which will be new container of the content
+   *                       of aLeftContent.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<JoinNodesResult, nsresult>
+  JoinNodesWithTransaction(nsIContent& aLeftContent, nsIContent& aRightContent);
+
+  /**
+   * JoinNearestEditableNodesWithTransaction() joins two editable nodes which
+   * are themselves or the nearest editable node of aLeftNode and aRightNode.
+   * XXX This method's behavior is odd.  For example, if user types Backspace
+   *     key at the second editable paragraph in this case:
+   *     <div contenteditable>
+   *       <p>first editable paragraph</p>
+   *       <p contenteditable="false">non-editable paragraph</p>
+   *       <p>second editable paragraph</p>
+   *     </div>
+   *     The first editable paragraph's content will be moved into the second
+   *     editable paragraph and the non-editable paragraph becomes the first
+   *     paragraph of the editor.  I don't think that it's expected behavior of
+   *     any users...
+   *
+   * @param aLeftNode   The node which will be removed.
+   * @param aRightNode  The node which will be inserted the content of
+   *                    aLeftNode.
+   * @param aNewFirstChildOfRightNode
+   *                    [out] The point at the first child of aRightNode.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  JoinNearestEditableNodesWithTransaction(
+      nsIContent& aLeftNode, nsIContent& aRightNode,
+      EditorDOMPoint* aNewFirstChildOfRightNode);
+
+  /**
+   * ReplaceContainerAndCloneAttributesWithTransaction() creates new element
+   * whose name is aTagName, copies all attributes from aOldContainer to the
+   * new element, moves all children in aOldContainer to the new element, then,
+   * removes aOldContainer from the DOM tree.
+   *
+   * @param aOldContainer       The element node which should be replaced
+   *                            with new element.
+   * @param aTagName            The name of new element node.
+   */
+  [[nodiscard]] inline MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  ReplaceContainerAndCloneAttributesWithTransaction(Element& aOldContainer,
+                                                    const nsAtom& aTagName);
+
+  /**
+   * ReplaceContainerWithTransaction() creates new element whose name is
+   * aTagName, sets aAttributes of the new element to aAttributeValue, moves
+   * all children in aOldContainer to the new element, then, removes
+   * aOldContainer from the DOM tree.
+   *
+   * @param aOldContainer       The element node which should be replaced
+   *                            with new element.
+   * @param aTagName            The name of new element node.
+   * @param aAttribute          Attribute name to be set to the new element.
+   * @param aAttributeValue     Attribute value to be set to aAttribute.
+   */
+  [[nodiscard]] inline MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  ReplaceContainerWithTransaction(Element& aOldContainer,
+                                  const nsAtom& aTagName,
+                                  const nsAtom& aAttribute,
+                                  const nsAString& aAttributeValue);
+
+  /**
+   * ReplaceContainerWithTransaction() creates new element whose name is
+   * aTagName, moves all children in aOldContainer to the new element, then,
+   * removes aOldContainer from the DOM tree.
+   *
+   * @param aOldContainer       The element node which should be replaced
+   *                            with new element.
+   * @param aTagName            The name of new element node.
+   */
+  [[nodiscard]] inline MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  ReplaceContainerWithTransaction(Element& aOldContainer,
+                                  const nsAtom& aTagName);
+
+  /**
+   * RemoveContainerWithTransaction() removes aElement from the DOM tree and
+   * moves all its children to the parent of aElement.
+   *
+   * @param aElement            The element to be removed.
+   * @return                    A suggestion point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  RemoveContainerWithTransaction(Element& aElement);
+
+  /**
+   * InsertContainerWithTransaction() creates new element whose name is
+   * aWrapperTagName, moves aContentToBeWrapped into the new element, then,
+   * inserts the new element into where aContentToBeWrapped was.
+   * NOTE: This method does not check if aContentToBeWrapped is valid child
+   * of the new element.  So, callers need to guarantee it.
+   *
+   * @param aContentToBeWrapped The content which will be wrapped with new
+   *                            element.
+   * @param aWrapperTagName     Element name of new element which will wrap
+   *                            aContent and be inserted into where aContent
+   *                            was.
+   * @param aInitializer        A callback to initialize new element before
+   *                            inserting to the DOM tree.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  InsertContainerWithTransaction(
+      nsIContent& aContentToBeWrapped, const nsAtom& aWrapperTagName,
+      const InitializeInsertingElement& aInitializer = DoNothingForNewElement);
+
+  /**
+   * MoveNodeWithTransaction() moves aContentToMove to aPointToInsert.
+   *
+   * @param aContentToMove  The node to be moved.
+   * @param aPointToInsert  The point where aContentToMove will be inserted.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<MoveNodeResult, nsresult>
+  MoveNodeWithTransaction(nsIContent& aContentToMove,
+                          const EditorDOMPoint& aPointToInsert);
+
+  /**
+   * MoveNodeToEndWithTransaction() moves aContentToMove to end of
+   * aNewContainer.
+   *
+   * @param aContentToMove  The node to be moved.
+   * @param aNewContainer   The new container which will contain aContentToMove
+   *                        as its last child.
+   */
+  [[nodiscard]] inline MOZ_CAN_RUN_SCRIPT Result<MoveNodeResult, nsresult>
+  MoveNodeToEndWithTransaction(nsIContent& aContentToMove,
+                               nsINode& aNewContainer);
+
+  /**
+   * MoveNodeOrChildrenWithTransaction() moves aContent to aPointToInsert.  If
+   * cannot insert aContent due to invalid relation, moves only its children
+   * recursively and removes aContent from the DOM tree.
+   *
+   * @param aContent            Content which should be moved.
+   * @param aPointToInsert      The point to be inserted aContent or its
+   *                            descendants.
+   * @param aPreserveWhiteSpaceStyle
+   *                            If yes and if it's possible to keep white-space
+   *                            style, this method will set `style` attribute to
+   *                            moving node or creating new <span> element.
+   * @param aRemoveIfCommentNode
+   *                            If yes, this removes a comment node instead of
+   *                            moving it to the destination.  Note that this
+   *                            does not remove comment nodes in moving nodes
+   *                            because it requires additional scan.
+   */
+  enum class PreserveWhiteSpaceStyle { No, Yes };
+  friend std::ostream& operator<<(
+      std::ostream& aStream,
+      const PreserveWhiteSpaceStyle aPreserveWhiteSpaceStyle);
+  enum class RemoveIfCommentNode { No, Yes };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<MoveNodeResult, nsresult>
+  MoveNodeOrChildrenWithTransaction(
+      nsIContent& aContentToMove, const EditorDOMPoint& aPointToInsert,
+      PreserveWhiteSpaceStyle aPreserveWhiteSpaceStyle,
+      RemoveIfCommentNode aRemoveIfCommentNode);
+
+  /**
+   * CanMoveNodeOrChildren() returns true if
+   * `MoveNodeOrChildrenWithTransaction()` can move or delete at least a
+   * descendant of aElement into aNewContainer.  I.e., when this returns true,
+   * `MoveNodeOrChildrenWithTransaction()` must return "handled".
+   */
+  Result<bool, nsresult> CanMoveNodeOrChildren(
+      const nsIContent& aContent, const nsINode& aNewContainer) const;
+
+  /**
+   * MoveChildrenWithTransaction() moves the children of aElement to
+   * aPointToInsert.  If cannot insert some children due to invalid relation,
+   * calls MoveNodeOrChildrenWithTransaction() to remove the children but keep
+   * moving its children.
+   *
+   * @param aElement            Container element whose children should be
+   *                            moved.
+   * @param aPointToInsert      The point to be inserted children of aElement
+   *                            or its descendants.
+   * @param aPreserveWhiteSpaceStyle
+   *                            If yes and if it's possible to keep white-space
+   *                            style, this method will set `style` attribute to
+   *                            moving node or creating new <span> element.
+   * @param aRemoveIfCommentNode
+   *                            If yes, this removes a comment node instead of
+   *                            moving it to the destination.  Note that this
+   *                            does not remove comment nodes in moving nodes
+   *                            because it requires additional scan.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<MoveNodeResult, nsresult>
+  MoveChildrenWithTransaction(Element& aElement,
+                              const EditorDOMPoint& aPointToInsert,
+                              PreserveWhiteSpaceStyle aPreserveWhiteSpaceStyle,
+                              RemoveIfCommentNode aRemoveIfCommentNode);
+
+  /**
+   * CanMoveChildren() returns true if `MoveChildrenWithTransaction()` can move
+   * at least a descendant of aElement into aNewContainer.  I.e., when this
+   * returns true, `MoveChildrenWithTransaction()` return "handled".
+   */
+  Result<bool, nsresult> CanMoveChildren(const Element& aElement,
+                                         const nsINode& aNewContainer) const;
+
+  /**
+   * MoveAllChildren() moves all children of aContainer to before
+   * aPointToInsert.GetChild().
+   * See explanation of MoveChildrenBetween() for the detail of the behavior.
+   *
+   * @param aContainer          The container node whose all children should
+   *                            be moved.
+   * @param aPointToInsert      The insertion point.  The container must not
+   *                            be a data node like a text node.
+   */
+  [[nodiscard]] nsresult MoveAllChildren(
+      nsINode& aContainer, const EditorRawDOMPoint& aPointToInsert);
+
+  /**
+   * MoveChildrenBetween() moves all children between aFirstChild and aLastChild
+   * to before aPointToInsert.GetChild(). If some children are moved to
+   * different container while this method moves other children, they are just
+   * ignored. If the child node referred by aPointToInsert is moved to different
+   * container while this method moves children, returns error.
+   *
+   * @param aFirstChild         The first child which should be moved to
+   *                            aPointToInsert.
+   * @param aLastChild          The last child which should be moved.  This
+   *                            must be a sibling of aFirstChild and it should
+   *                            be positioned after aFirstChild in the DOM tree
+   *                            order.
+   * @param aPointToInsert      The insertion point.  The container must not
+   *                            be a data node like a text node.
+   */
+  [[nodiscard]] nsresult MoveChildrenBetween(
+      nsIContent& aFirstChild, nsIContent& aLastChild,
+      const EditorRawDOMPoint& aPointToInsert);
+
+  /**
+   * MovePreviousSiblings() moves all siblings before aChild (i.e., aChild
+   * won't be moved) to before aPointToInsert.GetChild().
+   * See explanation of MoveChildrenBetween() for the detail of the behavior.
+   *
+   * @param aChild              The node which is next sibling of the last
+   *                            node to be moved.
+   * @param aPointToInsert      The insertion point.  The container must not
+   *                            be a data node like a text node.
+   */
+  [[nodiscard]] nsresult MovePreviousSiblings(
+      nsIContent& aChild, const EditorRawDOMPoint& aPointToInsert);
+
+  /**
+   * MoveInclusiveNextSiblings() moves aChild and all siblings after it to
+   * before aPointToInsert.GetChild().
+   * See explanation of MoveChildrenBetween() for the detail of the behavior.
+   *
+   * @param aChild              The node which is first node to be moved.
+   * @param aPointToInsert      The insertion point.  The container must not
+   *                            be a data node like a text node.
+   */
+  [[nodiscard]] nsresult MoveInclusiveNextSiblings(
+      nsIContent& aChild, const EditorRawDOMPoint& aPointToInsert);
+
+  /**
+   * SplitNodeWithTransaction() creates a transaction to create a new node
+   * (left node) identical to an existing node (right node), and split the
+   * contents between the same point in both nodes, then, execute the
+   * transaction.
+   *
+   * @param aStartOfRightNode   The point to split.  Its container will be
+   *                            the right node, i.e., become the new node's
+   *                            next sibling.  And the point will be start
+   *                            of the right node.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  SplitNodeWithTransaction(const EditorDOMPoint& aStartOfRightNode);
+
+  /**
+   * SplitNodeDeepWithTransaction() splits aMostAncestorToSplit deeply.
+   *
+   * @param aMostAncestorToSplit        The most ancestor node which should be
+   *                                    split.
+   * @param aStartOfDeepestRightNode    The start point of deepest right node.
+   *                                    This point must be descendant of
+   *                                    aMostAncestorToSplit.
+   * @param aSplitAtEdges               Whether the caller allows this to
+   *                                    create empty container element when
+   *                                    split point is start or end of an
+   *                                    element.
+   * @return                            SplitPoint() returns split point in
+   *                                    aMostAncestorToSplit.  The point must
+   *                                    be good to insert something if the
+   *                                    caller want to do it.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitNodeResult, nsresult>
+  SplitNodeDeepWithTransaction(nsIContent& aMostAncestorToSplit,
+                               const EditorDOMPoint& aDeepestStartOfRightNode,
+                               SplitAtEdges aSplitAtEdges);
+
+  /**
+   * RemoveEmptyInclusiveAncestorInlineElements() removes empty inclusive
+   * ancestor inline elements in inclusive ancestor block element of aContent.
+   *
+   * @param aContent    Must be an empty content.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  RemoveEmptyInclusiveAncestorInlineElements(nsIContent& aContent);
+
+  /**
+   * DeleteTextAndNormalizeSurroundingWhiteSpaces() deletes text between
+   * aStartToDelete and immediately before aEndToDelete and return new caret
+   * position.  If surrounding characters are white-spaces, this normalize them
+   * too.  Finally, inserts `<br>` element if it's required.
+   * Note that if you wants only normalizing white-spaces, you can set same
+   * point to both aStartToDelete and aEndToDelete.  Then, this tries to
+   * normalize white-space sequence containing previous character of
+   * aStartToDelete.
+   */
+  enum class DeleteDirection {
+    Forward,
+    Backward,
+  };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CaretPoint, nsresult>
+  DeleteTextAndNormalizeSurroundingWhiteSpaces(
+      const EditorDOMPointInText& aStartToDelete,
+      const EditorDOMPointInText& aEndToDelete,
+      TreatEmptyTextNodes aTreatEmptyTextNodes,
+      DeleteDirection aDeleteDirection);
+
+  /**
+   * ExtendRangeToDeleteWithNormalizingWhiteSpaces() is a helper method of
+   * DeleteTextAndNormalizeSurroundingWhiteSpaces().  This expands
+   * aStartToDelete and/or aEndToDelete if there are white-spaces which need
+   * normalizing.
+   *
+   * @param aStartToDelete      [In/Out] Start to delete.  If this point
+   *                            follows white-spaces, this may be modified.
+   * @param aEndToDelete        [In/Out] Next point of last content to be
+   *                            deleted.  If this point is a white-space,
+   *                            this may be modified.
+   * @param aNormalizedWhiteSpacesInStartNode
+   *                            [Out] If container text node of aStartToDelete
+   *                            should be modified, this offers new string
+   *                            in the range in the text node.
+   * @param aNormalizedWhiteSpacesInEndNode
+   *                            [Out] If container text node of aEndToDelete
+   *                            is different from the container of
+   *                            aStartToDelete and it should be modified, this
+   *                            offers new string in the range in the text node.
+   */
+  void ExtendRangeToDeleteWithNormalizingWhiteSpaces(
+      EditorDOMPointInText& aStartToDelete, EditorDOMPointInText& aEndToDelete,
+      nsAString& aNormalizedWhiteSpacesInStartNode,
+      nsAString& aNormalizedWhiteSpacesInEndNode) const;
+
+  /**
+   * CharPointType let the following helper methods of
+   * ExtendRangeToDeleteWithNormalizingWhiteSpaces() know what type of
+   * character will be previous or next char point after deletion.
+   */
+  enum class CharPointType {
+    TextEnd,  // Start or end of the text (hardline break or replaced inline
+              // element)
+    ASCIIWhiteSpace,   // One of ASCII white-spaces (collapsible white-space)
+    NoBreakingSpace,   // NBSP
+    VisibleChar,       // Non-white-space characters
+    PreformattedChar,  // Any character except a linefeed in a preformatted
+                       // node.
+    PreformattedLineBreak,  // Preformatted linebreak
+  };
+
+  /**
+   * GetPreviousCharPointType() and GetCharPointType() get type of
+   * previous/current char point from current DOM tree.  In other words, if the
+   * point will be deleted, you cannot use these methods.
+   */
+  template <typename EditorDOMPointType>
+  static CharPointType GetPreviousCharPointType(
+      const EditorDOMPointType& aPoint);
+  template <typename EditorDOMPointType>
+  static CharPointType GetCharPointType(const EditorDOMPointType& aPoint);
+
+  /**
+   * CharPointData let the following helper methods of
+   * ExtendRangeToDeleteWithNormalizingWhiteSpaces() know what type of
+   * character will be previous or next char point and the point is
+   * in same or different text node after deletion.
+   */
+  class MOZ_STACK_CLASS CharPointData final {
+   public:
+    static CharPointData InDifferentTextNode(CharPointType aCharPointType) {
+      CharPointData result;
+      result.mIsInDifferentTextNode = true;
+      result.mType = aCharPointType;
+      return result;
+    }
+    static CharPointData InSameTextNode(CharPointType aCharPointType) {
+      CharPointData result;
+      // Let's mark this as in different text node if given one indicates
+      // that there is end of text because it means that adjacent content
+      // from point of text node view is another element.
+      result.mIsInDifferentTextNode = aCharPointType == CharPointType::TextEnd;
+      result.mType = aCharPointType;
+      return result;
+    }
+
+    bool AcrossTextNodeBoundary() const { return mIsInDifferentTextNode; }
+    bool IsCollapsibleWhiteSpace() const {
+      return mType == CharPointType::ASCIIWhiteSpace ||
+             mType == CharPointType::NoBreakingSpace;
+    }
+    CharPointType Type() const { return mType; }
+
+   private:
+    CharPointData() = default;
+
+    CharPointType mType;
+    bool mIsInDifferentTextNode;
+  };
+
+  /**
+   * GetPreviousCharPointDataForNormalizingWhiteSpaces() and
+   * GetInclusiveNextCharPointDataForNormalizingWhiteSpaces() is helper methods
+   * of ExtendRangeToDeleteWithNormalizingWhiteSpaces().  This retrieves
+   * previous or inclusive next editable char point and returns its data.
+   */
+  CharPointData GetPreviousCharPointDataForNormalizingWhiteSpaces(
+      const EditorDOMPointInText& aPoint) const;
+  CharPointData GetInclusiveNextCharPointDataForNormalizingWhiteSpaces(
+      const EditorDOMPointInText& aPoint) const;
+
+  /**
+   * GenerateWhiteSpaceSequence() generates white-space sequence which won't
+   * be collapsed.
+   *
+   * @param aResult             [out] White space sequence which won't be
+   *                            collapsed, but wrapable.
+   * @param aLength             Length of generating white-space sequence.
+   *                            Must be 1 or larger.
+   * @param aPreviousCharPointData
+   *                            Specify the previous char point where it'll be
+   *                            inserted.  Currently, for keepin this method
+   *                            simple, does not support to generate a part
+   *                            of white-space sequence in a text node.  So,
+   *                            if the type is white-space, it must indicate
+   *                            different text nodes white-space.
+   * @param aNextCharPointData  Specify the next char point where it'll be
+   *                            inserted.  Same as aPreviousCharPointData,
+   *                            this must node indidate white-space in same
+   *                            text node.
+   */
+  static void GenerateWhiteSpaceSequence(
+      nsAString& aResult, uint32_t aLength,
+      const CharPointData& aPreviousCharPointData,
+      const CharPointData& aNextCharPointData);
+
+  /**
+   * ComputeTargetRanges() computes actual delete ranges which will be deleted
+   * unless the following `beforeinput` event is canceled.
+   *
+   * @param aDirectionAndAmount         The direction and amount of deletion.
+   * @param aRangesToDelete             [In/Out] The ranges to be deleted,
+   *                                    typically, initialized with the
+   *                                    selection ranges.  This may be modified
+   *                                    if selection ranges should be extened.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  ComputeTargetRanges(nsIEditor::EDirection aDirectionAndAmount,
+                      AutoRangeArray& aRangesToDelete) const;
+
+  /**
+   * This method handles "delete selection" commands.
+   *
+   * @param aDirectionAndAmount Direction of the deletion.
+   * @param aStripWrappers      Must be eStrip or eNoStrip.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  HandleDeleteSelection(nsIEditor::EDirection aDirectionAndAmount,
+                        nsIEditor::EStripWrappers aStripWrappers) final;
+
+  class AutoDeleteRangesHandler;
+  class AutoMoveOneLineHandler;
+
+  /**
+   * DeleteMostAncestorMailCiteElementIfEmpty() deletes most ancestor
+   * mail cite element (`<blockquote type="cite">` or
+   * `<span _moz_quote="true">`, the former can be created with middle click
+   * paste with `Control` or `Command` even in the web) of aContent if it
+   * becomes empty.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteMostAncestorMailCiteElementIfEmpty(nsIContent& aContent);
+
+  /**
+   * LiftUpListItemElement() moves aListItemElement outside its parent.
+   * If it's in a middle of a list element, the parent list element is split
+   * before aListItemElement.  Then, moves aListItemElement to before its
+   * parent list element.  I.e., moves aListItemElement between the 2 list
+   * elements if original parent was split.  Then, if new parent becomes not a
+   * list element, the list item element is removed and its contents are moved
+   * to where the list item element was.  If aListItemElement becomse not a
+   * child of list element, its contents are unwrapped from aListItemElement.
+   *
+   * @param aListItemElement    Must be a <li>, <dt> or <dd> element.
+   * @param aLiftUpFromAllParentListElements
+   *                            If Yes, this method calls itself recursively
+   *                            to unwrap the contents in aListItemElement
+   *                            from any ancestor list elements.
+   *                            XXX This checks only direct parent of list
+   *                                elements.  Therefore, if a parent list
+   *                                element in a list item element, the
+   *                                list item element and its list element
+   *                                won't be unwrapped.
+   */
+  enum class LiftUpFromAllParentListElements { Yes, No };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult LiftUpListItemElement(
+      dom::Element& aListItemElement,
+      LiftUpFromAllParentListElements aLiftUpFromAllParentListElements);
+
+  /**
+   * DestroyListStructureRecursively() destroys the list structure of
+   * aListElement recursively.
+   * If aListElement has <li>, <dl> or <dt> as a child, the element is removed
+   * but its descendants are moved to where the list item element was.
+   * If aListElement has another <ul>, <ol> or <dl> as a child, this method is
+   * called recursively.
+   * If aListElement has other nodes as its child, they are just removed.
+   * Finally, aListElement is removed. and its all children are moved to
+   * where the aListElement was.
+   *
+   * @param aListElement        A <ul>, <ol> or <dl> element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  DestroyListStructureRecursively(Element& aListElement);
+
+  /**
+   * RemoveListAtSelectionAsSubAction() removes list elements and list item
+   * elements at Selection.  And move contents in them where the removed list
+   * was.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  RemoveListAtSelectionAsSubAction(const Element& aEditingHost);
+
+  /**
+   * ChangeMarginStart() changes margin of aElement to indent or outdent.
+   * If it's rtl text, margin-right will be changed.  Otherwise, margin-left.
+   * XXX This is not aware of vertical writing-mode.
+   *
+   * @param aElement            The element whose start margin should be
+   *                            changed.
+   * @param aChangeMargin       Whether increase or decrease the margin.
+   * @param aEditingHost        The editing host.
+   * @return                    May suggest a suggest point to put caret.
+   */
+  enum class ChangeMargin { Increase, Decrease };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  ChangeMarginStart(Element& aElement, ChangeMargin aChangeMargin,
+                    const Element& aEditingHost);
+
+  /**
+   * HandleCSSIndentAroundRanges() indents around aRanges with CSS.
+   *
+   * @param aRanges             The ranges where the content should be indented.
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult HandleCSSIndentAroundRanges(
+      AutoRangeArray& aRanges, const Element& aEditingHost);
+
+  /**
+   * HandleCSSIndentAroundRanges() indents around aRanges with HTML.
+   *
+   * @param aRanges             The ranges where the content should be indented.
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult HandleHTMLIndentAroundRanges(
+      AutoRangeArray& aRanges, const Element& aEditingHost);
+
+  /**
+   * HandleIndentAtSelection() indents around Selection with HTML or CSS.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  // TODO: Make this take AutoRangeArray instead of retrieving `Selection`
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  HandleIndentAtSelection(const Element& aEditingHost);
+
+  /**
+   * OutdentPartOfBlock() outdents the nodes between aStartOfOutdent and
+   * aEndOfOutdent.  This splits the range off from aBlockElement first.
+   * Then, removes the middle element if aIsBlockIndentedWithCSS is false.
+   * Otherwise, decreases the margin of the middle element.
+   *
+   * @param aBlockElement       A block element which includes both
+   *                            aStartOfOutdent and aEndOfOutdent.
+   * @param aStartOfOutdent     First node which is descendant of
+   *                            aBlockElement will be outdented.
+   * @param aEndOfOutdent       Last node which is descandant of
+   *                            aBlockElement will be outdented.
+   * @param aBlockIndentedWith  `CSS` if aBlockElement is indented with
+   *                            CSS margin property.
+   *                            `HTML` if aBlockElement is `<blockquote>`
+   *                            or something.
+   * @param aEditingHost        The editing host.
+   * @return                    The left content is new created element
+   *                            splitting before aStartOfOutdent.
+   *                            The right content is existing element.
+   *                            The middle content is outdented element
+   *                            if aBlockIndentedWith is `CSS`.
+   *                            Otherwise, nullptr.
+   */
+  enum class BlockIndentedWith { CSS, HTML };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitRangeOffFromNodeResult, nsresult>
+  OutdentPartOfBlock(Element& aBlockElement, nsIContent& aStartOfOutdent,
+                     nsIContent& aEndOfOutdent,
+                     BlockIndentedWith aBlockIndentedWith,
+                     const Element& aEditingHost);
+
+  /**
+   * HandleOutdentAtSelectionInternal() outdents contents around Selection.
+   * This method creates AutoSelectionRestorer.  Therefore, each caller
+   * needs to check if the editor is still available even if this returns
+   * NS_OK.
+   * NOTE: Call `HandleOutdentAtSelection()` instead.
+   *
+   * @param aEditingHost        The editing host.
+   * @return                    The left content is left content of last
+   *                            outdented element.
+   *                            The right content is right content of last
+   *                            outdented element.
+   *                            The middle content is middle content of last
+   *                            outdented element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<SplitRangeOffFromNodeResult, nsresult>
+  HandleOutdentAtSelectionInternal(const Element& aEditingHost);
+
+  /**
+   * HandleOutdentAtSelection() outdents contents around Selection.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  HandleOutdentAtSelection(const Element& aEditingHost);
+
+  /**
+   * AlignBlockContentsWithDivElement() sets align attribute of <div> element
+   * which is only child of aBlockElement to aAlignType.  If aBlockElement
+   * has 2 or more children or does not have a `<div>` element, inserts a
+   * new `<div>` element into aBlockElement and move all children of
+   * aBlockElement into the new `<div>` element.
+   *
+   * @param aBlockElement       The element node whose contents should be
+   *                            aligned to aAlignType.  This should be
+   *                            an element which can have `<div>` element
+   *                            as its child.
+   * @param aAlignType          New value of align attribute of `<div>`
+   *                            element.
+   * @return                    A candidate position to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  AlignBlockContentsWithDivElement(Element& aBlockElement,
+                                   const nsAString& aAlignType);
+
+  /**
+   * AlignContentsInAllTableCellsAndListItems() calls
+   * AlignBlockContentsWithDivElement() for aligning contents in every list
+   * item element and table cell element in aElement.
+   *
+   * @param aElement            The node which is or whose descendants should
+   *                            be aligned to aAlignType.
+   * @param aAlignType          New value of `align` attribute of `<div>`.
+   * @return                    A candidate position to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  AlignContentsInAllTableCellsAndListItems(dom::Element& aElement,
+                                           const nsAString& aAlignType);
+
+  /**
+   * MakeTransitionList() detects all the transitions in the array, where a
+   * transition means that adjacent nodes in the array don't have the same
+   * parent.
+   */
+  static void MakeTransitionList(
+      const nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      nsTArray<bool>& aTransitionArray);
+
+  /**
+   * EnsureHardLineBeginsWithFirstChildOf() inserts `<br>` element before
+   * first child of aRemovingContainerElement if it will not be start of a
+   * hard line after removing aRemovingContainerElement.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  EnsureHardLineBeginsWithFirstChildOf(Element& aRemovingContainerElement);
+
+  /**
+   * EnsureHardLineEndsWithLastChildOf() inserts `<br>` element after last
+   * child of aRemovingContainerElement if it will not be end of a hard line
+   * after removing aRemovingContainerElement.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  EnsureHardLineEndsWithLastChildOf(Element& aRemovingContainerElement);
+
+  /**
+   * RemoveAlignFromDescendants() removes align attributes, text-align
+   * properties and <center> elements in aElement.
+   *
+   * @param aElement    Alignment information of the node and/or its
+   *                    descendants will be removed.
+   *                    NOTE: aElement must not be a `<table>` element.
+   * @param aAlignType  New align value to be set only when it's in
+   *                    CSS mode and this method meets <table> or <hr>.
+   *                    XXX This is odd and not clear when you see caller of
+   *                    this method.  Do you have better idea?
+   * @param aEditTarget If `OnlyDescendantsExceptTable`, modifies only
+   *                    descendants of aElement.
+   *                    If `NodeAndDescendantsExceptTable`, modifies `aElement`
+   *                    and its descendants.
+   * @return            A candidate point to put caret.
+   */
+  enum class EditTarget {
+    OnlyDescendantsExceptTable,
+    NodeAndDescendantsExceptTable
+  };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  RemoveAlignFromDescendants(Element& aElement, const nsAString& aAlignType,
+                             EditTarget aEditTarget);
+
+  /**
+   * SetBlockElementAlign() resets `align` attribute, `text-align` property
+   * of descendants of aBlockOrHRElement except `<table>` element descendants.
+   * Then, set `align` attribute or `text-align` property of aBlockOrHRElement.
+   *
+   * @param aBlockOrHRElement   The element whose contents will be aligned.
+   *                            This must be a block element or `<hr>` element.
+   *                            If we're not in CSS mode, this element has
+   *                            to support `align` attribute (i.e.,
+   *                            `HTMLEditUtils::SupportsAlignAttr()` must
+   *                            return true).
+   * @param aAlignType          Boundary or "center" which contents should be
+   *                            aligned on.
+   * @param aEditTarget         If `OnlyDescendantsExceptTable`, modifies only
+   *                            descendants of aBlockOrHRElement.
+   *                            If `NodeAndDescendantsExceptTable`, modifies
+   *                            aBlockOrHRElement and its descendants.
+   * @return                    A candidate point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  SetBlockElementAlign(Element& aBlockOrHRElement, const nsAString& aAlignType,
+                       EditTarget aEditTarget);
+
+  /**
+   * InsertDivElementToAlignContents() inserts a new <div> element (which has
+   * only a padding <br> element) to aPointToInsert for a placeholder whose
+   * contents will be aligned.
+   *
+   * @param aPointToInsert      A point to insert new empty <div>.
+   * @param aAlignType          New align attribute value where the contents
+   *                            should be aligned to.
+   * @param aEditingHost        The editing host.
+   * @return                    New <div> element which has only a padding <br>
+   *                            element and is styled to align contents.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  InsertDivElementToAlignContents(const EditorDOMPoint& aPointToInsert,
+                                  const nsAString& aAlignType,
+                                  const Element& aEditingHost);
+
+  /**
+   * AlignNodesAndDescendants() make contents of nodes in aArrayOfContents and
+   * their descendants aligned to aAlignType.
+   *
+   * @param aAlignType          New align attribute value where the contents
+   *                            should be aligned to.
+   * @param aEditingHost        The editing host.
+   * @return                    Last created <div> element which should contain
+   *                            caret and candidate position which may be
+   *                            outside the <div> element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  AlignNodesAndDescendants(
+      nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      const nsAString& aAlignType, const Element& aEditingHost);
+
+  /**
+   * AlignContentsAtRanges() aligns contents around aRanges to aAlignType.
+   *
+   * @param aRanges             The ranges where should be aligned.
+   * @param aAlignType          New align attribute value where the contents
+   *                            should be aligned to.
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  AlignContentsAtRanges(AutoRangeArray& aRanges, const nsAString& aAlignType,
+                        const Element& aEditingHost);
+
+  /**
+   * AlignAsSubAction() handles "align" command with `Selection`.
+   *
+   * @param aAlignType          New align attribute value where the contents
+   *                            should be aligned to.
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  AlignAsSubAction(const nsAString& aAlignType, const Element& aEditingHost);
+
+  /**
+   * AdjustCaretPositionAndEnsurePaddingBRElement() may adjust caret
+   * position to nearest editable content and if padding `<br>` element is
+   * necessary at caret position, this creates it.
+   *
+   * @param aDirectionAndAmount Direction of the edit action.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  AdjustCaretPositionAndEnsurePaddingBRElement(
+      nsIEditor::EDirection aDirectionAndAmount);
+
+  /**
+   * EnsureSelectionInBodyOrDocumentElement() collapse `Selection` to the
+   * primary `<body>` element or document element when `Selection` crosses
+   * `<body>` element's boundary.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  EnsureSelectionInBodyOrDocumentElement();
+
+  /**
+   * InsertBRElementToEmptyListItemsAndTableCellsInRange() inserts
+   * `<br>` element into empty list item or table cell elements between
+   * aStartRef and aEndRef.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  InsertBRElementToEmptyListItemsAndTableCellsInRange(
+      const RawRangeBoundary& aStartRef, const RawRangeBoundary& aEndRef);
+
+  /**
+   * RemoveEmptyNodesIn() removes all empty nodes in aRange.  However, if
+   * mail-cite node has only a `<br>` element, the node will be removed
+   * but <br> element is moved to where the mail-cite node was.
+   * XXX This method is expensive if aRange is too wide and may remove
+   *     unexpected empty element, e.g., it was created by JS, but we haven't
+   *     touched it.  Cannot we remove this method and make guarantee that
+   *     empty nodes won't be created?
+   *
+   * @param aRange      Must be positioned.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  RemoveEmptyNodesIn(const EditorDOMRange& aRange);
+
+  /**
+   * SetSelectionInterlinePosition() may set interline position if caret is
+   * positioned around `<br>` or block boundary.  Don't call this when
+   * `Selection` is not collapsed.
+   */
+  void SetSelectionInterlinePosition();
+
+  /**
+   * Called by `HTMLEditor::OnEndHandlingTopLevelEditSubAction()`.  This may
+   * adjust Selection, remove unnecessary empty nodes, create `<br>` elements
+   * if needed, etc.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  OnEndHandlingTopLevelEditSubActionInternal();
+
+  /**
+   * MoveSelectedContentsToDivElementToMakeItAbsolutePosition() looks for
+   * a `<div>` element in selection first.  If not, creates new `<div>`
+   * element.  Then, move all selected contents into the target `<div>`
+   * element.
+   * Note that this creates AutoSelectionRestorer.  Therefore, callers need
+   * to check whether we have been destroyed even when this returns NS_OK.
+   *
+   * @param aTargetElement      Returns target `<div>` element which should be
+   *                            changed to absolute positioned.
+   * @param aEditingHost        The editing host.
+   */
+  // TODO: Rewrite this with `Result<RefPtr<Element>, nsresult>`.
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  MoveSelectedContentsToDivElementToMakeItAbsolutePosition(
+      RefPtr<Element>* aTargetElement, const Element& aEditingHost);
+
+  /**
+   * SetSelectionToAbsoluteAsSubAction() move selected contents to first
+   * selected `<div>` element or newly created `<div>` element and make
+   * the `<div>` element positioned absolutely.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  SetSelectionToAbsoluteAsSubAction(const Element& aEditingHost);
+
+  /**
+   * SetSelectionToStaticAsSubAction() sets the `position` property of a
+   * selection parent's block whose `position` is `absolute` to `static`.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  SetSelectionToStaticAsSubAction();
+
+  /**
+   * AddZIndexAsSubAction() adds aChange to `z-index` of nearest parent
+   * absolute-positioned element from current selection.
+   *
+   * @param aChange     Amount to change `z-index`.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  AddZIndexAsSubAction(int32_t aChange);
+
+  /**
+   * OnDocumentModified() is called when editor content is changed.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult OnDocumentModified();
+
+ protected:  // Called by helper classes.
+  MOZ_CAN_RUN_SCRIPT void OnStartToHandleTopLevelEditSubAction(
+      EditSubAction aTopLevelEditSubAction,
+      nsIEditor::EDirection aDirectionOfTopLevelEditSubAction,
+      ErrorResult& aRv) final;
+  MOZ_CAN_RUN_SCRIPT nsresult OnEndHandlingTopLevelEditSubAction() final;
+
+ protected:  // Shouldn't be used by friend classes
+  virtual ~HTMLEditor();
+
+  /**
+   * InitEditorContentAndSelection() may insert `<br>` elements and padding
+   * `<br>` elements if they are required for `<body>` or document element.
+   * And collapse selection at the end if there is no selection ranges.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InitEditorContentAndSelection();
+
+  /**
+   * Collapse `Selection` to the last leaf content of the <body> or the document
+   * element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  CollapseSelectionToEndOfLastLeafNodeOfDocument() const;
+
+  MOZ_CAN_RUN_SCRIPT nsresult SelectAllInternal() final;
+
+  [[nodiscard]] Element* ComputeEditingHostInternal(
+      const nsIContent* aContent, LimitInBodyElement aLimitInBodyElement) const;
+
+  /**
+   * Creates a range with just the supplied node and appends that to the
+   * selection.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  AppendContentToSelectionAsRange(nsIContent& aContent);
+
+  /**
+   * When you are using AppendContentToSelectionAsRange(), call this first to
+   * start a new selection.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult ClearSelection();
+
+  /**
+   * SelectContentInternal() sets Selection to aContentToSelect to
+   * aContentToSelect + 1 in parent of aContentToSelect.
+   *
+   * @param aContentToSelect    The content which should be selected.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  SelectContentInternal(nsIContent& aContentToSelect);
+
+  /**
+   * GetInclusiveAncestorByTagNameAtSelection() looks for an element node whose
+   * name matches aTagName from anchor node of Selection to <body> element.
+   *
+   * @param aTagName        The tag name which you want to look for.
+   *                        Must not be nsGkAtoms::_empty.
+   *                        If nsGkAtoms::list, the result may be <ul>, <ol> or
+   *                        <dl> element.
+   *                        If nsGkAtoms::td, the result may be <td> or <th>.
+   *                        If nsGkAtoms::href, the result may be <a> element
+   *                        which has "href" attribute with non-empty value.
+   *                        If nsGkAtoms::anchor, the result may be <a> which
+   *                        has "name" attribute with non-empty value.
+   * @return                If an element which matches aTagName, returns
+   *                        an Element.  Otherwise, nullptr.
+   */
+  Element* GetInclusiveAncestorByTagNameAtSelection(
+      const nsStaticAtom& aTagName) const;
+
+  /**
+   * GetInclusiveAncestorByTagNameInternal() looks for an element node whose
+   * name matches aTagName from aNode to <body> element.
+   *
+   * @param aTagName        The tag name which you want to look for.
+   *                        Must not be nsGkAtoms::_empty.
+   *                        If nsGkAtoms::list, the result may be <ul>, <ol> or
+   *                        <dl> element.
+   *                        If nsGkAtoms::td, the result may be <td> or <th>.
+   *                        If nsGkAtoms::href, the result may be <a> element
+   *                        which has "href" attribute with non-empty value.
+   *                        If nsGkAtoms::anchor, the result may be <a> which
+   *                        has "name" attribute with non-empty value.
+   * @param aContent        Start node to look for the element.  This should
+   *                        not be an orphan node.
+   * @return                If an element which matches aTagName, returns
+   *                        an Element.  Otherwise, nullptr.
+   */
+  Element* GetInclusiveAncestorByTagNameInternal(
+      const nsStaticAtom& aTagName, const nsIContent& aContent) const;
+
+  /**
+   * GetSelectedElement() returns a "selected" element node.  "selected" means:
+   * - there is only one selection range
+   * - the range starts from an element node or in an element
+   * - the range ends at immediately after same element
+   * - and the range does not include any other element nodes.
+   * Additionally, only when aTagName is nsGkAtoms::href, this thinks that an
+   * <a> element which has non-empty "href" attribute includes the range, the
+   * <a> element is selected.
+   *
+   * NOTE: This method is implementation of nsIHTMLEditor.getSelectedElement()
+   * and comm-central depends on this behavior.  Therefore, if you need to use
+   * this method internally but you need to change, perhaps, you should create
+   * another method for avoiding breakage of comm-central apps.
+   *
+   * @param aTagName    The atom of tag name in lower case.  Set this to
+   *                    result  of EditorUtils::GetTagNameAtom() if you have a
+   *                    tag name with nsString.
+   *                    If nullptr, this returns any element node or nullptr.
+   *                    If nsGkAtoms::href, this returns an <a> element which
+   *                    has non-empty "href" attribute or nullptr.
+   *                    If nsGkAtoms::anchor, this returns an <a> element which
+   *                    has non-empty "name" attribute or nullptr.
+   *                    Otherwise, returns an element node whose name is
+   *                    same as aTagName or nullptr.
+   * @param aRv         Returns error code.
+   * @return            A "selected" element.
+   */
+  already_AddRefed<Element> GetSelectedElement(const nsAtom* aTagName,
+                                               ErrorResult& aRv);
+
+  /**
+   * GetFirstTableRowElement() returns the first <tr> element in the most
+   * nearest ancestor of aTableOrElementInTable or itself.
+   * When aTableOrElementInTable is neither <table> nor in a <table> element,
+   * returns NS_ERROR_FAILURE. However, if <table> does not have <tr> element,
+   * returns nullptr.
+   *
+   * @param aTableOrElementInTable      <table> element or another element.
+   *                                    If this is a <table> element, returns
+   *                                    first <tr> element in it.  Otherwise,
+   *                                    returns first <tr> element in nearest
+   *                                    ancestor <table> element.
+   */
+  Result<RefPtr<Element>, nsresult> GetFirstTableRowElement(
+      const Element& aTableOrElementInTable) const;
+
+  /**
+   * GetNextTableRowElement() returns next <tr> element of aTableRowElement.
+   * This won't cross <table> element boundary but may cross table section
+   * elements like <tbody>.
+   * Note that if given element is <tr> but there is no next <tr> element, this
+   * returns nullptr but does not return error.
+   *
+   * @param aTableRowElement    A <tr> element.
+   */
+  Result<RefPtr<Element>, nsresult> GetNextTableRowElement(
+      const Element& aTableRowElement) const;
+
+  struct CellData;
+
+  /**
+   * CellIndexes store both row index and column index of a table cell.
+   */
+  struct MOZ_STACK_CLASS CellIndexes final {
+    int32_t mRow;
+    int32_t mColumn;
+
+    /**
+     * This constructor initializes mRowIndex and mColumnIndex with indexes of
+     * aCellElement.
+     *
+     * @param aCellElement      An <td> or <th> element.
+     */
+    MOZ_CAN_RUN_SCRIPT CellIndexes(Element& aCellElement, PresShell* aPresShell)
+        : mRow(-1), mColumn(-1) {
+      Update(aCellElement, aPresShell);
+    }
+
+    /**
+     * Update mRowIndex and mColumnIndex with indexes of aCellElement.
+     *
+     * @param                   See above.
+     */
+    MOZ_CAN_RUN_SCRIPT void Update(Element& aCellElement,
+                                   PresShell* aPresShell);
+
+    /**
+     * This constructor initializes mRowIndex and mColumnIndex with indexes of
+     * cell element which contains anchor of Selection.
+     *
+     * @param aHTMLEditor       The editor which creates the instance.
+     * @param aSelection        The Selection for the editor.
+     */
+    MOZ_CAN_RUN_SCRIPT CellIndexes(HTMLEditor& aHTMLEditor,
+                                   Selection& aSelection)
+        : mRow(-1), mColumn(-1) {
+      Update(aHTMLEditor, aSelection);
+    }
+
+    /**
+     * Update mRowIndex and mColumnIndex with indexes of cell element which
+     * contains anchor of Selection.
+     *
+     * @param                   See above.
+     */
+    MOZ_CAN_RUN_SCRIPT void Update(HTMLEditor& aHTMLEditor,
+                                   Selection& aSelection);
+
+    bool operator==(const CellIndexes& aOther) const {
+      return mRow == aOther.mRow && mColumn == aOther.mColumn;
+    }
+    bool operator!=(const CellIndexes& aOther) const {
+      return mRow != aOther.mRow || mColumn != aOther.mColumn;
+    }
+
+    [[nodiscard]] bool isErr() const { return mRow < 0 || mColumn < 0; }
+
+   private:
+    CellIndexes() : mRow(-1), mColumn(-1) {}
+    CellIndexes(int32_t aRowIndex, int32_t aColumnIndex)
+        : mRow(aRowIndex), mColumn(aColumnIndex) {}
+
+    friend struct CellData;
+  };
+
+  struct MOZ_STACK_CLASS CellData final {
+    MOZ_KNOWN_LIVE RefPtr<Element> mElement;
+    // Current indexes which this is initialized with.
+    CellIndexes mCurrent;
+    // First column/row indexes of the cell.  When current position is spanned
+    // from other column/row, this value becomes different from mCurrent.
+    CellIndexes mFirst;
+    // Computed rowspan/colspan values which are specified to the cell.
+    // Note that if the cell has larger rowspan/colspan value than actual
+    // table size, these values are the larger values.
+    int32_t mRowSpan = -1;
+    int32_t mColSpan = -1;
+    // Effective rowspan/colspan value at the index.  For example, if first
+    // cell element in first row has rowspan="3", then, if this is initialized
+    // with 0-0 indexes, effective rowspan is 3.  However, if this is
+    // initialized with 1-0 indexes, effective rowspan is 2.
+    int32_t mEffectiveRowSpan = -1;
+    int32_t mEffectiveColSpan = -1;
+    // mIsSelected is set to true if mElement itself or its parent <tr> or
+    // <table> is selected.  Otherwise, e.g., the cell just contains selection
+    // range, this is set to false.
+    bool mIsSelected = false;
+
+    CellData() = delete;
+
+    /**
+     * This returns an instance which is initialized with a <table> element and
+     * both row and column index to specify a cell element.
+     */
+    [[nodiscard]] static CellData AtIndexInTableElement(
+        const HTMLEditor& aHTMLEditor, const Element& aTableElement,
+        int32_t aRowIndex, int32_t aColumnIndex);
+    [[nodiscard]] static CellData AtIndexInTableElement(
+        const HTMLEditor& aHTMLEditor, const Element& aTableElement,
+        const CellIndexes& aIndexes) {
+      MOZ_ASSERT(!aIndexes.isErr());
+      return AtIndexInTableElement(aHTMLEditor, aTableElement, aIndexes.mRow,
+                                   aIndexes.mColumn);
+    }
+
+    /**
+     * Treated as error if fails to compute current index or first index of the
+     * cell.  Note that even if the cell is not found due to no corresponding
+     * frame at current index, it's not an error situation.
+     */
+    [[nodiscard]] bool isOk() const { return !isErr(); }
+    [[nodiscard]] bool isErr() const { return mFirst.isErr(); }
+
+    /**
+     * FailedOrNotFound() returns true if this failed to initialize/update
+     * or succeeded but found no cell element.
+     */
+    [[nodiscard]] bool FailedOrNotFound() const { return isErr() || !mElement; }
+
+    /**
+     * IsSpannedFromOtherRowOrColumn(), IsSpannedFromOtherColumn and
+     * IsSpannedFromOtherRow() return true if there is no cell element
+     * at the index because of spanning from other row and/or column.
+     */
+    [[nodiscard]] bool IsSpannedFromOtherRowOrColumn() const {
+      return mElement && mCurrent != mFirst;
+    }
+    [[nodiscard]] bool IsSpannedFromOtherColumn() const {
+      return mElement && mCurrent.mColumn != mFirst.mColumn;
+    }
+    [[nodiscard]] bool IsSpannedFromOtherRow() const {
+      return mElement && mCurrent.mRow != mFirst.mRow;
+    }
+    [[nodiscard]] bool IsNextColumnSpannedFromOtherColumn() const {
+      return mElement && mCurrent.mColumn + 1 < NextColumnIndex();
+    }
+
+    /**
+     * NextColumnIndex() and NextRowIndex() return column/row index of
+     * next cell.  Note that this does not check whether there is next
+     * cell or not actually.
+     */
+    [[nodiscard]] int32_t NextColumnIndex() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mCurrent.mColumn + mEffectiveColSpan;
+    }
+    [[nodiscard]] int32_t NextRowIndex() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mCurrent.mRow + mEffectiveRowSpan;
+    }
+
+    /**
+     * LastColumnIndex() and LastRowIndex() return column/row index of
+     * column/row which is spanned by the cell.
+     */
+    [[nodiscard]] int32_t LastColumnIndex() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return NextColumnIndex() - 1;
+    }
+    [[nodiscard]] int32_t LastRowIndex() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return NextRowIndex() - 1;
+    }
+
+    /**
+     * NumberOfPrecedingColmuns() and NumberOfPrecedingRows() return number of
+     * preceding columns/rows if current index is spanned from other column/row.
+     * Otherwise, i.e., current point is not spanned form other column/row,
+     * returns 0.
+     */
+    [[nodiscard]] int32_t NumberOfPrecedingColmuns() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mCurrent.mColumn - mFirst.mColumn;
+    }
+    [[nodiscard]] int32_t NumberOfPrecedingRows() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mCurrent.mRow - mFirst.mRow;
+    }
+
+    /**
+     * NumberOfFollowingColumns() and NumberOfFollowingRows() return
+     * number of remaining columns/rows if the cell spans to other
+     * column/row.
+     */
+    [[nodiscard]] int32_t NumberOfFollowingColumns() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mEffectiveColSpan - 1;
+    }
+    [[nodiscard]] int32_t NumberOfFollowingRows() const {
+      if (NS_WARN_IF(FailedOrNotFound())) {
+        return -1;
+      }
+      return mEffectiveRowSpan - 1;
+    }
+
+   private:
+    explicit CellData(int32_t aCurrentRowIndex, int32_t aCurrentColumnIndex,
+                      int32_t aFirstRowIndex, int32_t aFirstColumnIndex)
+        : mCurrent(aCurrentRowIndex, aCurrentColumnIndex),
+          mFirst(aFirstRowIndex, aFirstColumnIndex) {}
+    explicit CellData(Element& aElement, int32_t aRowIndex,
+                      int32_t aColumnIndex, nsTableCellFrame& aTableCellFrame,
+                      nsTableWrapperFrame& aTableWrapperFrame);
+
+    [[nodiscard]] static CellData Error(int32_t aRowIndex,
+                                        int32_t aColumnIndex) {
+      return CellData(aRowIndex, aColumnIndex, -1, -1);
+    }
+    [[nodiscard]] static CellData NotFound(int32_t aRowIndex,
+                                           int32_t aColumnIndex) {
+      return CellData(aRowIndex, aColumnIndex, aRowIndex, aColumnIndex);
+    }
+  };
+
+  /**
+   * TableSize stores and computes number of rows and columns of a <table>
+   * element.
+   */
+  struct MOZ_STACK_CLASS TableSize final {
+    int32_t mRowCount;
+    int32_t mColumnCount;
+
+    TableSize() = delete;
+
+    /**
+     * @param aHTMLEditor               The editor which creates the instance.
+     * @param aTableOrElementInTable    If a <table> element, computes number
+     *                                  of rows and columns of it.
+     *                                  If another element in a <table> element,
+     *                                  computes number of rows and columns
+     *                                  of nearest ancestor <table> element.
+     *                                  Otherwise, i.e., non-<table> element
+     *                                  not in <table>, returns error.
+     */
+    [[nodiscard]] static Result<TableSize, nsresult> Create(
+        HTMLEditor& aHTMLEditor, Element& aTableOrElementInTable);
+
+    [[nodiscard]] bool IsEmpty() const { return !mRowCount || !mColumnCount; }
+
+   private:
+    TableSize(int32_t aRowCount, int32_t aColumCount)
+        : mRowCount(aRowCount), mColumnCount(aColumCount) {}
+  };
+
+  /**
+   * GetTableCellElementAt() returns a <td> or <th> element of aTableElement
+   * if there is a cell at the indexes.
+   *
+   * @param aTableElement       Must be a <table> element.
+   * @param aCellIndexes        Indexes of cell which you want.
+   *                            If rowspan and/or colspan is specified 2 or
+   *                            larger, any indexes are allowed to retrieve
+   *                            the cell in the area.
+   * @return                    The cell element if there is in the <table>.
+   *                            Returns nullptr without error if the indexes
+   *                            are out of bounds.
+   */
+  [[nodiscard]] inline Element* GetTableCellElementAt(
+      Element& aTableElement, const CellIndexes& aCellIndexes) const;
+  [[nodiscard]] Element* GetTableCellElementAt(Element& aTableElement,
+                                               int32_t aRowIndex,
+                                               int32_t aColumnIndex) const;
+
+  /**
+   * GetSelectedOrParentTableElement() returns <td>, <th>, <tr> or <table>
+   * element:
+   *   #1 if the first selection range selects a cell, returns it.
+   *   #2 if the first selection range does not select a cell and
+   *      the selection anchor refers a <table>, returns it.
+   *   #3 if the first selection range does not select a cell and
+   *      the selection anchor refers a <tr>, returns it.
+   *   #4 if the first selection range does not select a cell and
+   *      the selection anchor refers a <td>, returns it.
+   *   #5 otherwise, nearest ancestor <td> or <th> element of the
+   *      selection anchor if there is.
+   * In #1 and #4, *aIsCellSelected will be set to true (i.e,, when
+   * a selection range selects a cell element).
+   */
+  Result<RefPtr<Element>, nsresult> GetSelectedOrParentTableElement(
+      bool* aIsCellSelected = nullptr) const;
+
+  /**
+   * GetFirstSelectedCellElementInTable() returns <td> or <th> element at
+   * first selection (using GetSelectedOrParentTableElement).  If found cell
+   * element is not in <table> or <tr> element, this returns nullptr.
+   */
+  Result<RefPtr<Element>, nsresult> GetFirstSelectedCellElementInTable() const;
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult HandlePaste(
+      AutoEditActionDataSetter& aEditActionData, int32_t aClipboardType) final;
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult HandlePasteAsQuotation(
+      AutoEditActionDataSetter& aEditActionData, int32_t aClipboardType) final;
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  HandlePasteTransferable(AutoEditActionDataSetter& aEditActionData,
+                          nsITransferable& aTransferable) final;
+
+  /**
+   * PasteInternal() pasts text with replacing selected content.
+   * This tries to dispatch ePaste event first.  If its defaultPrevent() is
+   * called, this does nothing but returns NS_OK.
+   *
+   * @param aClipboardType      nsIClipboard::kGlobalClipboard or
+   *                            nsIClipboard::kSelectionClipboard.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult PasteInternal(int32_t aClipboardType);
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  InsertWithQuotationsAsSubAction(const nsAString& aQuotedText) final;
+
+  /**
+   * InsertAsCitedQuotationInternal() inserts a <blockquote> element whose
+   * cite attribute is aCitation and whose content is aQuotedText.
+   * Note that this shouldn't be called when IsPlaintextMailComposer() is true.
+   *
+   * @param aQuotedText     HTML source if aInsertHTML is true.  Otherwise,
+   *                        plain text.  This is inserted into new <blockquote>
+   *                        element.
+   * @param aCitation       cite attribute value of new <blockquote> element.
+   * @param aInsertHTML     true if aQuotedText should be treated as HTML
+   *                        source.
+   *                        false if aQuotedText should be treated as plain
+   *                        text.
+   * @param aNodeInserted   [OUT] The new <blockquote> element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertAsCitedQuotationInternal(
+      const nsAString& aQuotedText, const nsAString& aCitation,
+      bool aInsertHTML, nsINode** aNodeInserted);
+
+  /**
+   * InsertNodeIntoProperAncestorWithTransaction() attempts to insert aNode
+   * into the document, at aPointToInsert.  Checks with strict dtd to see if
+   * containment is allowed.  If not allowed, will attempt to find a parent
+   * in the parent hierarchy of aPointToInsert.GetContainer() that will accept
+   * aNode as a child.  If such a parent is found, will split the document
+   * tree from aPointToInsert up to parent, and then insert aNode.
+   * aPointToInsert is then adjusted to point to the actual location that
+   * aNode was inserted at.  aSplitAtEdges specifies if the splitting process
+   * is allowed to result in empty nodes.
+   *
+   * @param aContent          The content node to insert.
+   * @param aPointToInsert    Insertion point.
+   * @param aSplitAtEdges     Splitting can result in empty nodes?
+   */
+  template <typename NodeType>
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT
+      Result<CreateNodeResultBase<NodeType>, nsresult>
+      InsertNodeIntoProperAncestorWithTransaction(
+          NodeType& aContent, const EditorDOMPoint& aPointToInsert,
+          SplitAtEdges aSplitAtEdges);
+
+  /**
+   * InsertTextWithQuotationsInternal() replaces selection with new content.
+   * First, this method splits aStringToInsert to multiple chunks which start
+   * with non-linebreaker except first chunk and end with a linebreaker except
+   * last chunk.  Then, each chunk starting with ">" is inserted after wrapping
+   * with <span _moz_quote="true">, and each chunk not starting with ">" is
+   * inserted as normal text.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  InsertTextWithQuotationsInternal(const nsAString& aStringToInsert);
+
+  /**
+   * ReplaceContainerWithTransactionInternal() is implementation of
+   * ReplaceContainerWithTransaction() and
+   * ReplaceContainerAndCloneAttributesWithTransaction().
+   *
+   * @param aOldContainer       The element which will be replaced with new
+   *                            element.
+   * @param aTagName            The name of new element node.
+   * @param aAttribute          Attribute name which will be set to the new
+   *                            element.  This will be ignored if
+   *                            aCloneAllAttributes is set to true.
+   * @param aAttributeValue     Attribute value which will be set to
+   *                            aAttribute.
+   * @param aCloneAllAttributes If true, all attributes of aOldContainer will
+   *                            be copied to the new element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  ReplaceContainerWithTransactionInternal(Element& aOldContainer,
+                                          const nsAtom& aTagName,
+                                          const nsAtom& aAttribute,
+                                          const nsAString& aAttributeValue,
+                                          bool aCloneAllAttributes);
+
+  /**
+   * DeleteSelectionAndCreateElement() creates a element whose name is aTag.
+   * And insert it into the DOM tree after removing the selected content.
+   *
+   * @param aTag                The element name to be created.
+   * @param aInitializer        A function to initialize the new element before
+   *                            or after (depends on the pref) connecting the
+   *                            element into the DOM tree. Note that this should
+   *                            not touch outside given element because doing it
+   *                            would break range updater's result.
+   */
+  MOZ_CAN_RUN_SCRIPT Result<RefPtr<Element>, nsresult>
+  DeleteSelectionAndCreateElement(
+      nsAtom& aTag,
+      const InitializeInsertingElement& aInitializer = DoNothingForNewElement);
+
+  /**
+   * This method first deletes the selection, if it's not collapsed.  Then if
+   * the selection lies in a CharacterData node, it splits it.  If the
+   * selection is at this point collapsed in a CharacterData node, it's
+   * adjusted to be collapsed right before or after the node instead (which is
+   * always possible, since the node was split).
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult DeleteSelectionAndPrepareToCreateNode();
+
+  /**
+   * PrepareToInsertBRElement() returns a point where new <br> element should
+   * be inserted.  If aPointToInsert points middle of a text node, this method
+   * splits the text node and returns the point before right node.
+   *
+   * @param aPointToInsert      Candidate point to insert new <br> element.
+   * @return                    Computed point to insert new <br> element.
+   *                            If something failed, this return error.
+   */
+  MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult> PrepareToInsertBRElement(
+      const EditorDOMPoint& aPointToInsert);
+
+  /**
+   * IndentAsSubAction() indents the content around Selection.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  IndentAsSubAction(const Element& aEditingHost);
+
+  /**
+   * OutdentAsSubAction() outdents the content around Selection.
+   *
+   * @param aEditingHost        The editing host.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  OutdentAsSubAction(const Element& aEditingHost);
+
+  MOZ_CAN_RUN_SCRIPT nsresult LoadHTML(const nsAString& aInputString);
+
+  /**
+   * UpdateMetaCharsetWithTransaction() scans all <meta> elements in the
+   * document and if and only if there is a <meta> element having `httpEquiv`
+   * attribute and whose value includes `content-type`, updates its `content`
+   * attribute value to aCharacterSet.
+   */
+  MOZ_CAN_RUN_SCRIPT bool UpdateMetaCharsetWithTransaction(
+      Document& aDocument, const nsACString& aCharacterSet);
+
+  /**
+   * SetInlinePropertiesAsSubAction() stores new styles with
+   * mPendingStylesToApplyToNewContent if `Selection` is collapsed.  Otherwise,
+   * applying the styles to all selected contents.
+   *
+   * @param aStylesToSet        The styles which should be applied to the
+   *                            selected content.
+   */
+  template <size_t N>
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetInlinePropertiesAsSubAction(
+      const AutoTArray<EditorInlineStyleAndValue, N>& aStylesToSet);
+
+  /**
+   * SetInlinePropertiesAroundRanges() applying the styles to the ranges even if
+   * the ranges are collapsed.
+   */
+  template <size_t N>
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetInlinePropertiesAroundRanges(
+      AutoRangeArray& aRanges,
+      const AutoTArray<EditorInlineStyleAndValue, N>& aStylesToSet,
+      const Element& aEditingHost);
+
+  /**
+   * RemoveInlinePropertiesAsSubAction() removes specified styles from
+   * mPendingStylesToApplyToNewContent if `Selection` is collapsed.  Otherwise,
+   * removing the style.
+   *
+   * @param aStylesToRemove     Styles to remove from the selected contents.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult RemoveInlinePropertiesAsSubAction(
+      const nsTArray<EditorInlineStyle>& aStylesToRemove);
+
+  /**
+   * Helper method to call RemoveInlinePropertiesAsSubAction().  If you want to
+   * remove other elements to remove the style completely, this will append
+   * related elements of aStyleToRemove and aStyleToRemove itself to the array.
+   * E.g., nsGkAtoms::strong and nsGkAtoms::b will be appended if aStyleToRemove
+   * is nsGkAtoms::b.
+   */
+  void AppendInlineStyleAndRelatedStyle(
+      const EditorInlineStyle& aStyleToRemove,
+      nsTArray<EditorInlineStyle>& aStylesToRemove) const;
+
+  /**
+   * ReplaceHeadContentsWithSourceWithTransaction() replaces all children of
+   * <head> element with given source code.  This is undoable.
+   *
+   * @param aSourceToInsert     HTML source fragment to replace the children
+   *                            of <head> element.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult ReplaceHeadContentsWithSourceWithTransaction(
+      const nsAString& aSourceToInsert);
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetCSSBackgroundColorState(
+      bool* aMixed, nsAString& aOutColor, bool aBlockLevel);
+  nsresult GetHTMLBackgroundColorState(bool* aMixed, nsAString& outColor);
+
+  /**
+   * This sets background on the appropriate container element (table, cell,)
+   * or calls to set the page background.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetBlockBackgroundColorWithCSSAsSubAction(const nsAString& aColor);
+  MOZ_CAN_RUN_SCRIPT nsresult
+  SetHTMLBackgroundColorWithTransaction(const nsAString& aColor);
+
+  MOZ_CAN_RUN_SCRIPT_BOUNDARY void InitializeSelectionAncestorLimit(
+      nsIContent& aAncestorLimit) const final;
+
+  /**
+   * Make the given selection span the entire document.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SelectEntireDocument() final;
+
+  /**
+   * Use this to assure that selection is set after attribute nodes when
+   * trying to collapse selection at begining of a block node
+   * e.g., when setting at beginning of a table cell
+   * This will stop at a table, however, since we don't want to
+   * "drill down" into nested tables.
+   */
+  MOZ_CAN_RUN_SCRIPT void CollapseSelectionToDeepestNonTableFirstChild(
+      nsINode* aNode);
+  /**
+   * MaybeCollapseSelectionAtFirstEditableNode() may collapse selection at
+   * proper position to staring to edit.  If there is a non-editable node
+   * before any editable text nodes or inline elements which can have text
+   * nodes as their children, collapse selection at start of the editing
+   * host.  If there is an editable text node which is not collapsed, collapses
+   * selection at the start of the text node.  If there is an editable inline
+   * element which cannot have text nodes as its child, collapses selection at
+   * before the element node.  Otherwise, collapses selection at start of the
+   * editing host.
+   *
+   * @param aIgnoreIfSelectionInEditingHost
+   *                        This method does nothing if selection is in the
+   *                        editing host except if it's collapsed at start of
+   *                        the editing host.
+   *                        Note that if selection ranges were outside of
+   *                        current selection limiter, selection was collapsed
+   *                        at the start of the editing host therefore, if
+   *                        you call this with setting this to true, you can
+   *                        keep selection ranges if user has already been
+   *                        changed.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  MaybeCollapseSelectionAtFirstEditableNode(
+      bool aIgnoreIfSelectionInEditingHost) const;
+
+  class BlobReader final {
+    using AutoEditActionDataSetter = EditorBase::AutoEditActionDataSetter;
+
+   public:
+    MOZ_CAN_RUN_SCRIPT BlobReader(dom::BlobImpl* aBlob, HTMLEditor* aHTMLEditor,
+                                  SafeToInsertData aSafeToInsertData,
+                                  const EditorDOMPoint& aPointToInsert,
+                                  DeleteSelectedContent aDeleteSelectedContent);
+
+    NS_INLINE_DECL_CYCLE_COLLECTING_NATIVE_REFCOUNTING(BlobReader)
+    NS_DECL_CYCLE_COLLECTION_NATIVE_CLASS(BlobReader)
+
+    MOZ_CAN_RUN_SCRIPT nsresult OnResult(const nsACString& aResult);
+    nsresult OnError(const nsAString& aErrorName);
+
+   private:
+    ~BlobReader() = default;
+
+    RefPtr<dom::BlobImpl> mBlob;
+    RefPtr<HTMLEditor> mHTMLEditor;
+    RefPtr<dom::DataTransfer> mDataTransfer;
+    EditorDOMPoint mPointToInsert;
+    EditAction mEditAction;
+    SafeToInsertData mSafeToInsertData;
+    DeleteSelectedContent mDeleteSelectedContent;
+    bool mNeedsToDispatchBeforeInputEvent;
+  };
+
+  void CreateEventListeners() final;
+  nsresult InstallEventListeners() final;
+
+  bool ShouldReplaceRootElement() const;
+  MOZ_CAN_RUN_SCRIPT void NotifyRootChanged();
+  Element* GetBodyElement() const;
+
+  /**
+   * Get the focused node of this editor.
+   * @return    If the editor has focus, this returns the focused node.
+   *            Otherwise, returns null.
+   */
+  nsINode* GetFocusedNode() const;
+
+  already_AddRefed<Element> GetInputEventTargetElement() const final;
+
+  /**
+   * Return TRUE if aElement is a table-related elemet and caret was set.
+   */
+  MOZ_CAN_RUN_SCRIPT bool SetCaretInTableCell(dom::Element* aElement);
+
+  /**
+   * HandleTabKeyPressInTable() handles "Tab" key press in table if selection
+   * is in a `<table>` element.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditActionResult, nsresult>
+  HandleTabKeyPressInTable(WidgetKeyboardEvent* aKeyboardEvent);
+
+  /**
+   * InsertPosition is an enum to indicate where the method should insert to.
+   */
+  enum class InsertPosition {
+    // Before selected cell or a cell containing first selection range.
+    eBeforeSelectedCell,
+    // After selected cell or a cell containing first selection range.
+    eAfterSelectedCell,
+  };
+
+  /**
+   * InsertTableCellsWithTransaction() inserts <td> elements at aPointToInsert.
+   * Note that this simply inserts <td> elements, i.e., colspan and rowspan
+   * around the cell containing selection are not modified.  So, for example,
+   * adding a cell to rectangular table changes non-rectangular table.
+   * And if the cell containing selection is at left of row-spanning cell,
+   * it may be moved to right side of the row-spanning cell after inserting
+   * some cell elements before it.  Similarly, colspan won't be adjusted
+   * for keeping table rectangle.
+   * Finally, puts caret into previous cell of the insertion point or the
+   * first inserted cell if aPointToInsert is start of the row.
+   *
+   * @param aPointToInsert              The place to insert one or more cell
+   *                                    elements.  The container must be a
+   *                                    <tr> element.
+   * @param aNumberOfCellsToInsert      Number of cells to insert.
+   * @return                            The first inserted cell element and
+   *                                    start of the last inserted cell element
+   *                                    as a point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<CreateElementResult, nsresult>
+  InsertTableCellsWithTransaction(const EditorDOMPoint& aPointToInsert,
+                                  int32_t aNumberOfCellsToInsert);
+
+  /**
+   * InsertTableColumnsWithTransaction() inserts cell elements to every rows
+   * at same column index as the cell specified by aPointToInsert.
+   *
+   * @param aNumberOfColumnsToInsert    Number of columns to insert.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertTableColumnsWithTransaction(
+      const EditorDOMPoint& aPointToInsert, int32_t aNumberOfColumnsToInsert);
+
+  /**
+   * InsertTableRowsWithTransaction() inserts <tr> elements before or after
+   * aCellElement.  When aCellElement spans rows and aInsertPosition is
+   * eAfterSelectedCell, new rows will be inserted after the most-bottom row
+   * which contains the cell.
+   *
+   * @param aCellElement                The cell element pinting where this will
+   *                                    insert a row before or after.
+   * @param aNumberOfRowsToInsert       Number of rows to insert.
+   * @param aInsertPosition             Before or after the target cell which
+   *                                    contains first selection range.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult InsertTableRowsWithTransaction(
+      Element& aCellElement, int32_t aNumberOfRowsToInsert,
+      InsertPosition aInsertPosition);
+
+  /**
+   * Insert a new cell after or before supplied aCell.
+   * Optional: If aNewCell supplied, returns the newly-created cell (addref'd,
+   * of course)
+   * This doesn't change or use the current selection.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult InsertCell(Element* aCell, int32_t aRowSpan,
+                                         int32_t aColSpan, bool aAfter,
+                                         bool aIsHeader, Element** aNewCell);
+
+  /**
+   * DeleteSelectedTableColumnsWithTransaction() removes cell elements which
+   * belong to same columns of selected cell elements.
+   * If only one cell element is selected or first selection range is
+   * in a cell, removes cell elements which belong to same column.
+   * If 2 or more cell elements are selected, removes cell elements which
+   * belong to any of all selected columns.  In this case,
+   * aNumberOfColumnsToDelete is ignored.
+   * If there is no selection ranges, returns error.
+   * If selection is not in a cell element, this does not return error,
+   * just does nothing.
+   * WARNING: This does not remove <col> nor <colgroup> elements.
+   *
+   * @param aNumberOfColumnsToDelete    Number of columns to remove.  This is
+   *                                    ignored if 2 ore more cells are
+   *                                    selected.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteSelectedTableColumnsWithTransaction(int32_t aNumberOfColumnsToDelete);
+
+  /**
+   * DeleteTableColumnWithTransaction() removes cell elements which belong
+   * to the specified column.
+   * This method adjusts colspan attribute value if cells spanning the
+   * column to delete.
+   * WARNING: This does not remove <col> nor <colgroup> elements.
+   *
+   * @param aTableElement       The <table> element which contains the
+   *                            column which you want to remove.
+   * @param aRowIndex           Index of the column which you want to remove.
+   *                            0 is the first column.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult DeleteTableColumnWithTransaction(
+      Element& aTableElement, int32_t aColumnIndex);
+
+  /**
+   * DeleteSelectedTableRowsWithTransaction() removes <tr> elements.
+   * If only one cell element is selected or first selection range is
+   * in a cell, removes <tr> elements starting from a <tr> element
+   * containing the selected cell or first selection range.
+   * If 2 or more cell elements are selected, all <tr> elements
+   * which contains selected cell(s).  In this case, aNumberOfRowsToDelete
+   * is ignored.
+   * If there is no selection ranges, returns error.
+   * If selection is not in a cell element, this does not return error,
+   * just does nothing.
+   *
+   * @param aNumberOfRowsToDelete   Number of rows to remove.  This is ignored
+   *                                if 2 or more cells are selected.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteSelectedTableRowsWithTransaction(int32_t aNumberOfRowsToDelete);
+
+  /**
+   * DeleteTableRowWithTransaction() removes a <tr> element whose index in
+   * the <table> is aRowIndex.
+   * This method adjusts rowspan attribute value if the <tr> element contains
+   * cells which spans rows.
+   *
+   * @param aTableElement       The <table> element which contains the
+   *                            <tr> element which you want to remove.
+   * @param aRowIndex           Index of the <tr> element which you want to
+   *                            remove.  0 is the first row.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteTableRowWithTransaction(Element& aTableElement, int32_t aRowIndex);
+
+  /**
+   * DeleteTableCellWithTransaction() removes table cell elements.  If two or
+   * more cell elements are selected, this removes all selected cell elements.
+   * Otherwise, this removes some cell elements starting from selected cell
+   * element or a cell containing first selection range.  When this removes
+   * last cell element in <tr> or <table>, this removes the <tr> or the
+   * <table> too.  Note that when removing a cell causes number of its row
+   * becomes less than the others, this method does NOT fill the place with
+   * rowspan nor colspan.  This does not return error even if selection is not
+   * in cell element, just does nothing.
+   *
+   * @param aNumberOfCellsToDelete  Number of cells to remove.  This is ignored
+   *                                if 2 or more cells are selected.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteTableCellWithTransaction(int32_t aNumberOfCellsToDelete);
+
+  /**
+   * DeleteAllChildrenWithTransaction() removes all children of aElement from
+   * the tree.
+   *
+   * @param aElement        The element whose children you want to remove.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteAllChildrenWithTransaction(Element& aElement);
+
+  /**
+   * Move all contents from aCellToMerge into aTargetCell (append at end).
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult MergeCells(RefPtr<Element> aTargetCell,
+                                         RefPtr<Element> aCellToMerge,
+                                         bool aDeleteCellToMerge);
+
+  /**
+   * DeleteTableElementAndChildren() removes aTableElement (and its children)
+   * from the DOM tree with transaction.
+   *
+   * @param aTableElement   The <table> element which you want to remove.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  DeleteTableElementAndChildrenWithTransaction(Element& aTableElement);
+
+  MOZ_CAN_RUN_SCRIPT nsresult SetColSpan(Element* aCell, int32_t aColSpan);
+  MOZ_CAN_RUN_SCRIPT nsresult SetRowSpan(Element* aCell, int32_t aRowSpan);
+
+  /**
+   * Helper used to get nsTableWrapperFrame for a table.
+   */
+  static nsTableWrapperFrame* GetTableFrame(const Element* aTable);
+
+  /**
+   * GetNumberOfCellsInRow() returns number of actual cell elements in the row.
+   * If some cells appear by "rowspan" in other rows, they are ignored.
+   *
+   * @param aTableElement   The <table> element.
+   * @param aRowIndex       Valid row index in aTableElement.  This method
+   *                        counts cell elements in the row.
+   * @return                -1 if this meets unexpected error.
+   *                        Otherwise, number of cells which this method found.
+   */
+  int32_t GetNumberOfCellsInRow(Element& aTableElement, int32_t aRowIndex);
+
+  /**
+   * Test if all cells in row or column at given index are selected.
+   */
+  bool AllCellsInRowSelected(Element* aTable, int32_t aRowIndex,
+                             int32_t aNumberOfColumns);
+  bool AllCellsInColumnSelected(Element* aTable, int32_t aColIndex,
+                                int32_t aNumberOfRows);
+
+  bool IsEmptyCell(Element* aCell);
+
+  /**
+   * Most insert methods need to get the same basic context data.
+   * Any of the pointers may be null if you don't need that datum (for more
+   * efficiency).
+   * Input: *aCell is a known cell,
+   *        if null, cell is obtained from the anchor node of the selection.
+   * Returns NS_EDITOR_ELEMENT_NOT_FOUND if cell is not found even if aCell is
+   * null.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult GetCellContext(Element** aTable, Element** aCell,
+                                             nsINode** aCellParent,
+                                             int32_t* aCellOffset,
+                                             int32_t* aRowIndex,
+                                             int32_t* aColIndex);
+
+  nsresult GetCellSpansAt(Element* aTable, int32_t aRowIndex, int32_t aColIndex,
+                          int32_t& aActualRowSpan, int32_t& aActualColSpan);
+
+  MOZ_CAN_RUN_SCRIPT nsresult SplitCellIntoColumns(
+      Element* aTable, int32_t aRowIndex, int32_t aColIndex,
+      int32_t aColSpanLeft, int32_t aColSpanRight, Element** aNewCell);
+
+  MOZ_CAN_RUN_SCRIPT nsresult SplitCellIntoRows(
+      Element* aTable, int32_t aRowIndex, int32_t aColIndex,
+      int32_t aRowSpanAbove, int32_t aRowSpanBelow, Element** aNewCell);
+
+  MOZ_CAN_RUN_SCRIPT nsresult CopyCellBackgroundColor(Element* aDestCell,
+                                                      Element* aSourceCell);
+
+  /**
+   * Reduce rowspan/colspan when cells span into nonexistent rows/columns.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult FixBadRowSpan(Element* aTable, int32_t aRowIndex,
+                                            int32_t& aNewRowCount);
+  MOZ_CAN_RUN_SCRIPT nsresult FixBadColSpan(Element* aTable, int32_t aColIndex,
+                                            int32_t& aNewColCount);
+
+  /**
+   * XXX NormalizeTableInternal() is broken.  If it meets a cell which has
+   *     bigger or smaller rowspan or colspan than actual number of cells,
+   *     this always failed to scan the table.  Therefore, this does nothing
+   *     when the table should be normalized.
+   *
+   * @param aTableOrElementInTable  An element which is in a <table> element
+   *                                or <table> element itself.  Otherwise,
+   *                                this returns NS_OK but does nothing.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  NormalizeTableInternal(Element& aTableOrElementInTable);
+
+  /**
+   * Fallback method: Call this after using ClearSelection() and you
+   * failed to set selection to some other content in the document.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetSelectionAtDocumentStart();
+
+  // Methods for handling plaintext quotations
+  MOZ_CAN_RUN_SCRIPT nsresult PasteAsPlaintextQuotation(int32_t aSelectionType);
+
+  /**
+   * Insert a string as quoted text, replacing the selected text (if any).
+   * @param aQuotedText     The string to insert.
+   * @param aAddCites       Whether to prepend extra ">" to each line
+   *                        (usually true, unless those characters
+   *                        have already been added.)
+   * @return aNodeInserted  The node spanning the insertion, if applicable.
+   *                        If aAddCites is false, this will be null.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertAsPlaintextQuotation(
+      const nsAString& aQuotedText, bool aAddCites, nsINode** aNodeInserted);
+
+  /**
+   * InsertObject() inserts given object at aPointToInsert.
+   *
+   * @param aType one of kFileMime, kJPEGImageMime, kJPGImageMime,
+   *              kPNGImageMime, kGIFImageMime.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult InsertObject(
+      const nsACString& aType, nsISupports* aObject,
+      SafeToInsertData aSafeToInsertData, const EditorDOMPoint& aPointToInsert,
+      DeleteSelectedContent aDeleteSelectedContent);
+
+  class HTMLTransferablePreparer;
+  nsresult PrepareHTMLTransferable(nsITransferable** aTransferable) const;
+
+  enum class HavePrivateHTMLFlavor { No, Yes };
+  MOZ_CAN_RUN_SCRIPT nsresult InsertFromTransferableAtSelection(
+      nsITransferable* aTransferable, const nsAString& aContextStr,
+      const nsAString& aInfoStr, HavePrivateHTMLFlavor aHavePrivateHTMLFlavor);
+
+  /**
+   * InsertFromDataTransfer() is called only when user drops data into
+   * this editor.  Don't use this method for other purposes.
+   *
+   * @param aIndex index of aDataTransfer's item to insert.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult InsertFromDataTransfer(
+      const dom::DataTransfer* aDataTransfer, uint32_t aIndex,
+      nsIPrincipal* aSourcePrincipal, const EditorDOMPoint& aDroppedAt,
+      DeleteSelectedContent aDeleteSelectedContent);
+
+  static HavePrivateHTMLFlavor ClipboardHasPrivateHTMLFlavor(
+      nsIClipboard* clipboard);
+
+  /**
+   * CF_HTML:
+   * <https://docs.microsoft.com/en-us/windows/win32/dataxchg/html-clipboard-format>.
+   *
+   * @param[in]  aCfhtml a CF_HTML string as defined above.
+   * @param[out] aStuffToPaste the fragment, excluding context.
+   * @param[out] aCfcontext the context, excluding the fragment, including a
+   *                        marker (`kInsertionCookie`) indicating where the
+   *                        fragment begins.
+   */
+  nsresult ParseCFHTML(const nsCString& aCfhtml, char16_t** aStuffToPaste,
+                       char16_t** aCfcontext);
+
+  /**
+   * AutoHTMLFragmentBoundariesFixer fixes both edges of topmost child contents
+   * which are created with SubtreeContentIterator.
+   */
+  class MOZ_STACK_CLASS AutoHTMLFragmentBoundariesFixer final {
+   public:
+    /**
+     * @param aArrayOfTopMostChildContents
+     *                         [in/out] The topmost child contents which will be
+     *                         inserted into the DOM tree.  Both edges, i.e.,
+     *                         first node and last node in this array will be
+     *                         checked whether they can be inserted into
+     *                         another DOM tree.  If not, it'll replaces some
+     *                         orphan nodes around nodes with proper parent.
+     */
+    explicit AutoHTMLFragmentBoundariesFixer(
+        nsTArray<OwningNonNull<nsIContent>>& aArrayOfTopMostChildContents);
+
+   private:
+    /**
+     * EnsureBeginsOrEndsWithValidContent() replaces some nodes starting from
+     * start or end with proper element node if it's necessary.
+     * If first or last node of aArrayOfTopMostChildContents is in list and/or
+     * `<table>` element, looks for topmost list element or `<table>` element
+     * with `CollectTableAndAnyListElementsOfInclusiveAncestorsAt()` and
+     * `GetMostDistantAncestorListOrTableElement()`.  Then, checks
+     * whether some nodes are in aArrayOfTopMostChildContents are the topmost
+     * list/table element or its descendant and if so, removes the nodes from
+     * aArrayOfTopMostChildContents and inserts the list/table element instead.
+     * Then, aArrayOfTopMostChildContents won't start/end with list-item nor
+     * table cells.
+     */
+    enum class StartOrEnd { start, end };
+    void EnsureBeginsOrEndsWithValidContent(
+        StartOrEnd aStartOrEnd,
+        nsTArray<OwningNonNull<nsIContent>>& aArrayOfTopMostChildContents)
+        const;
+
+    /**
+     * CollectTableAndAnyListElementsOfInclusiveAncestorsAt() collects list
+     * elements and table related elements from the inclusive ancestors
+     * (https://dom.spec.whatwg.org/#concept-tree-inclusive-ancestor) of aNode.
+     */
+    static void CollectTableAndAnyListElementsOfInclusiveAncestorsAt(
+        nsIContent& aContent,
+        nsTArray<OwningNonNull<Element>>& aOutArrayOfListAndTableElements);
+
+    /**
+     * GetMostDistantAncestorListOrTableElement() returns a list or a
+     * `<table>` element which is in
+     * aInclusiveAncestorsTableOrListElements and they are actually
+     * valid ancestor of at least one of aArrayOfTopMostChildContents.
+     */
+    static Element* GetMostDistantAncestorListOrTableElement(
+        const nsTArray<OwningNonNull<nsIContent>>& aArrayOfTopMostChildContents,
+        const nsTArray<OwningNonNull<Element>>&
+            aInclusiveAncestorsTableOrListElements);
+
+    /**
+     * FindReplaceableTableElement() is a helper method of
+     * EnsureBeginsOrEndsWithValidContent().  If aNodeMaybeInTableElement is
+     * a descendant of aTableElement, returns aNodeMaybeInTableElement or its
+     * nearest ancestor whose tag name is `<td>`, `<th>`, `<tr>`, `<thead>`,
+     * `<tfoot>`, `<tbody>` or `<caption>`.
+     *
+     * @param aTableElement               Must be a `<table>` element.
+     * @param aContentMaybeInTableElement A node which may be in aTableElement.
+     */
+    Element* FindReplaceableTableElement(
+        Element& aTableElement, nsIContent& aContentMaybeInTableElement) const;
+
+    /**
+     * IsReplaceableListElement() is a helper method of
+     * EnsureBeginsOrEndsWithValidContent().  If aNodeMaybeInListElement is a
+     * descendant of aListElement, returns true.  Otherwise, false.
+     *
+     * @param aListElement                Must be a list element.
+     * @param aContentMaybeInListElement  A node which may be in aListElement.
+     */
+    bool IsReplaceableListElement(Element& aListElement,
+                                  nsIContent& aContentMaybeInListElement) const;
+  };
+
+  /**
+   * MakeDefinitionListItemWithTransaction() replaces parent list of current
+   * selection with <dl> or create new <dl> element and creates a definition
+   * list item whose name is aTagName.
+   *
+   * @param aTagName            Must be nsGkAtoms::dt or nsGkAtoms::dd.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  MakeDefinitionListItemWithTransaction(nsAtom& aTagName);
+
+  /**
+   * FormatBlockContainerAsSubAction() inserts a block element whose name
+   * is aTagName at selection.  If selection is not collapsed and aTagName is
+   * nsGkAtoms::normal or nsGkAtoms::_empty, this removes block containers.
+   *
+   * @param aTagName            A block level element name.  Must NOT be
+   *                            nsGkAtoms::dt nor nsGkAtoms::dd.
+   * @param aFormatBlockMode    Whether HTML formatBlock command or XUL
+   *                            paragraphState command.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult FormatBlockContainerAsSubAction(
+      const nsStaticAtom& aTagName, FormatBlockMode aFormatBlockMode);
+
+  /**
+   * Increase/decrease the font size of selection.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  IncrementOrDecrementFontSizeAsSubAction(FontSize aIncrementOrDecrement);
+
+  /**
+   * Wrap aContent in <big> or <small> element and make children of
+   * <font size=n> wrap with <big> or <small> too.  Note that if there is
+   * opposite element for aIncrementOrDecrement, their children will be just
+   * unwrapped.
+   *
+   * @param aDir        Whether increase or decrease the font size of aContent.
+   * @param aContent    The content node whose font size will be changed.
+   * @return            A suggest point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  SetFontSizeWithBigOrSmallElement(nsIContent& aContent,
+                                   FontSize aIncrementOrDecrement);
+
+  /**
+   * Adjust font size of font element children recursively with handling
+   * <big> and <small> elements.
+   *
+   * @param aDir        Whether increase or decrease the font size of aContent.
+   * @param aContent    The content node whose font size will be changed.
+   *                    All descendants will be handled recursively.
+   * @return            A suggest point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  SetFontSizeOfFontElementChildren(nsIContent& aContent,
+                                   FontSize aIncrementOrDecrement);
+
+  /**
+   * Get extended range to select element whose all children are selected by
+   * aRange.
+   */
+  EditorRawDOMRange GetExtendedRangeWrappingEntirelySelectedElements(
+      const EditorRawDOMRange& aRange) const;
+
+  /**
+   * Get extended range to select ancestor <a name> elements.
+   */
+  EditorRawDOMRange GetExtendedRangeWrappingNamedAnchor(
+      const EditorRawDOMRange& aRange) const;
+
+  // Declared in HTMLEditorNestedClasses.h and defined in HTMLStyleEditor.cpp
+  class AutoInlineStyleSetter;
+
+  /**
+   * RemoveStyleInside() removes elements which represent aStyleToRemove
+   * and removes CSS style.  This handles aElement and all its descendants
+   * (including leaf text nodes) recursively.
+   * TODO: Rename this to explain that this maybe remove aElement from the DOM
+   *       tree.
+   *
+   * @param aSpecifiedStyle  Whether the class and style attributes should
+   *                         be preserved or discarded.
+   * @return                 A suggest point to put caret.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  RemoveStyleInside(Element& aElement, const EditorInlineStyle& aStyleToRemove,
+                    SpecifiedStyle aSpecifiedStyle);
+
+  /**
+   * CollectEditableLeafTextNodes() collects text nodes in aElement.
+   */
+  void CollectEditableLeafTextNodes(
+      Element& aElement, nsTArray<OwningNonNull<Text>>& aLeafTextNodes) const;
+
+  /**
+   * IsRemovableParentStyleWithNewSpanElement() checks whether aStyle of parent
+   * block can be removed from aContent with creating `<span>` element.  Note
+   * that this does NOT check whether the specified style comes from parent
+   * block or not.
+   * XXX This may destroy the editor, but using `Result<bool, nsresult>`
+   *     is not reasonable because code for accessing the result becomes
+   *     messy.  However, anybody must forget to check `Destroyed()` after
+   *     calling this.  Which is the way to smart to make every caller
+   *     must check the editor state?
+   */
+  MOZ_CAN_RUN_SCRIPT Result<bool, nsresult>
+  IsRemovableParentStyleWithNewSpanElement(
+      nsIContent& aContent, const EditorInlineStyle& aStyle) const;
+
+  /**
+   * HasStyleOrIdOrClassAttribute() returns true when at least one of
+   * `style`, `id` or `class` attribute value of aElement is not empty.
+   */
+  static bool HasStyleOrIdOrClassAttribute(Element& aElement);
+
+  /**
+   * Whether the outer window of the DOM event target has focus or not.
+   */
+  bool OurWindowHasFocus() const;
+
+  class HTMLWithContextInserter;
+
+  /**
+   * This function is used to insert a string of HTML input optionally with some
+   * context information into the editable field.  The HTML input either comes
+   * from a transferable object created as part of a drop/paste operation, or
+   * from the InsertHTML method.  We may want the HTML input to be sanitized
+   * (for example, if it's coming from a transferable object), in which case
+   * aTrustedInput should be set to false, otherwise, the caller should set it
+   * to true, which means that the HTML will be inserted in the DOM verbatim.
+   */
+  enum class InlineStylesAtInsertionPoint {
+    Preserve,  // If you want the paste to be affected by local style, e.g.,
+               // for the insertHTML command, use "Preserve"
+    Clear,     // If you want the paste to be keep its own style, e.g., pasting
+               // from clipboard, use "Clear"
+  };
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult InsertHTMLWithContextAsSubAction(
+      const nsAString& aInputString, const nsAString& aContextStr,
+      const nsAString& aInfoStr, const nsAString& aFlavor,
+      SafeToInsertData aSafeToInsertData, const EditorDOMPoint& aPointToInsert,
+      DeleteSelectedContent aDeleteSelectedContent,
+      InlineStylesAtInsertionPoint aInlineStylesAtInsertionPoint);
+
+  /**
+   * sets the position of an element; warning it does NOT check if the
+   * element is already positioned or not and that's on purpose.
+   * @param aStyledElement      [IN] the element
+   * @param aX                  [IN] the x position in pixels.
+   * @param aY                  [IN] the y position in pixels.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetTopAndLeftWithTransaction(
+      nsStyledElement& aStyledElement, int32_t aX, int32_t aY);
+
+  /**
+   * Reset a selected cell or collapsed selection (the caret) after table
+   * editing.
+   *
+   * @param aTable      A table in the document.
+   * @param aRow        The row ...
+   * @param aCol        ... and column defining the cell where we will try to
+   *                    place the caret.
+   * @param aSelected   If true, we select the whole cell instead of setting
+   *                    caret.
+   * @param aDirection  If cell at (aCol, aRow) is not found, search for
+   *                    previous cell in the same column (aPreviousColumn) or
+   *                    row (ePreviousRow) or don't search for another cell
+   *                    (aNoSearch).  If no cell is found, caret is place just
+   *                    before table; and if that fails, at beginning of
+   *                    document.  Thus we generally don't worry about the
+   *                    return value and can use the
+   *                    AutoSelectionSetterAfterTableEdit stack-based object to
+   *                    insure we reset the caret in a table-editing method.
+   */
+  MOZ_CAN_RUN_SCRIPT void SetSelectionAfterTableEdit(Element* aTable,
+                                                     int32_t aRow, int32_t aCol,
+                                                     int32_t aDirection,
+                                                     bool aSelected);
+
+  void RemoveListenerAndDeleteRef(const nsAString& aEvent,
+                                  nsIDOMEventListener* aListener,
+                                  bool aUseCapture, ManualNACPtr aElement,
+                                  PresShell* aPresShell);
+  void DeleteRefToAnonymousNode(ManualNACPtr aContent, PresShell* aPresShell);
+
+  /**
+   * RefreshEditingUI() may refresh editing UIs for current Selection, focus,
+   * etc.  If this shows or hides some UIs, it causes reflow.  So, this is
+   * not safe method.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult RefreshEditingUI();
+
+  /**
+   * Returns the offset of an element's frame to its absolute containing block.
+   */
+  nsresult GetElementOrigin(Element& aElement, int32_t& aX, int32_t& aY);
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult GetPositionAndDimensions(
+      Element& aElement, int32_t& aX, int32_t& aY, int32_t& aW, int32_t& aH,
+      int32_t& aBorderLeft, int32_t& aBorderTop, int32_t& aMarginLeft,
+      int32_t& aMarginTop);
+
+  bool IsInObservedSubtree(nsIContent* aChild);
+
+  void UpdateRootElement();
+
+  /**
+   * SetAllResizersPosition() moves all resizers to proper position.
+   * If the resizers are hidden or replaced with another set of resizers
+   * while this is running, this returns error.  So, callers shouldn't
+   * keep handling the resizers if this returns error.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult SetAllResizersPosition();
+
+  /**
+   * Shows active resizers around an element's frame
+   * @param aResizedElement [IN] a DOM Element
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult ShowResizersInternal(Element& aResizedElement);
+
+  /**
+   * Hide resizers if they are visible.  If this is called while there is no
+   * visible resizers, this does not return error, but does nothing.
+   */
+  nsresult HideResizersInternal();
+
+  /**
+   * RefreshResizersInternal() moves resizers to proper position.  This does
+   * nothing if there is no resizing target.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult RefreshResizersInternal();
+
+  ManualNACPtr CreateResizer(int16_t aLocation, nsIContent& aParentContent);
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetAnonymousElementPositionWithoutTransaction(nsStyledElement& aStyledElement,
+                                                int32_t aX, int32_t aY);
+
+  ManualNACPtr CreateShadow(nsIContent& aParentContent,
+                            Element& aOriginalObject);
+
+  /**
+   * SetShadowPosition() moves the shadow element to proper position.
+   *
+   * @param aShadowElement      Must be mResizingShadow or mPositioningShadow.
+   * @param aElement            The element which has the shadow.
+   * @param aElementX           Left of aElement.
+   * @param aElementY           Top of aElement.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetShadowPosition(Element& aShadowElement, Element& aElement,
+                    int32_t aElementLeft, int32_t aElementTop);
+
+  ManualNACPtr CreateResizingInfo(nsIContent& aParentContent);
+  MOZ_CAN_RUN_SCRIPT nsresult SetResizingInfoPosition(int32_t aX, int32_t aY,
+                                                      int32_t aW, int32_t aH);
+
+  enum class ResizeAt {
+    eX,
+    eY,
+    eWidth,
+    eHeight,
+  };
+  [[nodiscard]] int32_t GetNewResizingIncrement(int32_t aX, int32_t aY,
+                                                ResizeAt aResizeAt) const;
+
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult StartResizing(Element& aHandle);
+  int32_t GetNewResizingX(int32_t aX, int32_t aY);
+  int32_t GetNewResizingY(int32_t aX, int32_t aY);
+  int32_t GetNewResizingWidth(int32_t aX, int32_t aY);
+  int32_t GetNewResizingHeight(int32_t aX, int32_t aY);
+  void HideShadowAndInfo();
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetFinalSizeWithTransaction(int32_t aX, int32_t aY);
+  void SetResizeIncrements(int32_t aX, int32_t aY, int32_t aW, int32_t aH,
+                           bool aPreserveRatio);
+
+  /**
+   * HideAnonymousEditingUIs() forcibly hides all editing UIs (resizers,
+   * inline-table-editing UI, absolute positioning UI).
+   */
+  void HideAnonymousEditingUIs();
+
+  /**
+   * HideAnonymousEditingUIsIfUnnecessary() hides all editing UIs if some of
+   * visible UIs are now unnecessary.
+   */
+  void HideAnonymousEditingUIsIfUnnecessary();
+
+  /**
+   * sets the z-index of an element.
+   * @param aElement [IN] the element
+   * @param aZorder  [IN] the z-index
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  SetZIndexWithTransaction(nsStyledElement& aElement, int32_t aZIndex);
+
+  /**
+   * shows a grabber attached to an arbitrary element. The grabber is an image
+   * positioned on the left hand side of the top border of the element. Draggin
+   * and dropping it allows to change the element's absolute position in the
+   * document. See chrome://editor/content/images/grabber.gif
+   * @param aElement [IN] the element
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult ShowGrabberInternal(Element& aElement);
+
+  /**
+   * Setting grabber to proper position for current mAbsolutelyPositionedObject.
+   * For example, while an element has grabber, the element may be resized
+   * or repositioned by script or something.  Then, you need to reset grabber
+   * position with this.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult RefreshGrabberInternal();
+
+  /**
+   * hide the grabber if it shown.
+   */
+  void HideGrabberInternal();
+
+  /**
+   * CreateGrabberInternal() creates a grabber for moving aParentContent.
+   * This sets mGrabber to the new grabber.  If this returns true, it's
+   * always non-nullptr.  Otherwise, i.e., the grabber is hidden during
+   * creation, this returns false.
+   */
+  bool CreateGrabberInternal(nsIContent& aParentContent);
+
+  MOZ_CAN_RUN_SCRIPT nsresult StartMoving();
+  MOZ_CAN_RUN_SCRIPT nsresult SetFinalPosition(int32_t aX, int32_t aY);
+  void SnapToGrid(int32_t& newX, int32_t& newY) const;
+  nsresult GrabberClicked();
+  MOZ_CAN_RUN_SCRIPT nsresult EndMoving();
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  GetTemporaryStyleForFocusedPositionedElement(Element& aElement,
+                                               nsAString& aReturn);
+
+  /**
+   * Shows inline table editing UI around a <table> element which contains
+   * aCellElement.  This returns error if creating UI is hidden during this,
+   * or detects another set of UI during this.  In such case, callers
+   * shouldn't keep handling anything for the UI.
+   *
+   * @param aCellElement    Must be an <td> or <th> element.
+   */
+  MOZ_CAN_RUN_SCRIPT nsresult
+  ShowInlineTableEditingUIInternal(Element& aCellElement);
+
+  /**
+   * Hide all inline table editing UI.
+   */
+  void HideInlineTableEditingUIInternal();
+
+  /**
+   * RefreshInlineTableEditingUIInternal() moves inline table editing UI to
+   * proper position.  This returns error if the UI is hidden or replaced
+   * during moving.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult
+  RefreshInlineTableEditingUIInternal();
+
+  enum class ContentNodeIs { Inserted, Appended };
+  MOZ_CAN_RUN_SCRIPT void DoContentInserted(nsIContent* aChild,
+                                            ContentNodeIs aContentNodeIs);
+
+  /**
+   * Returns an anonymous Element of type aTag,
+   * child of aParentContent. If aIsCreatedHidden is true, the class
+   * "hidden" is added to the created element. If aAnonClass is not
+   * the empty string, it becomes the value of the attribute "_moz_anonclass"
+   * @return a Element
+   * @param aTag             [IN] desired type of the element to create
+   * @param aParentContent   [IN] the parent node of the created anonymous
+   *                              element
+   * @param aAnonClass       [IN] contents of the _moz_anonclass attribute
+   * @param aIsCreatedHidden [IN] a boolean specifying if the class "hidden"
+   *                              is to be added to the created anonymous
+   *                              element
+   */
+  ManualNACPtr CreateAnonymousElement(nsAtom* aTag, nsIContent& aParentContent,
+                                      const nsAString& aAnonClass,
+                                      bool aIsCreatedHidden);
+
+  /**
+   * Reads a blob into memory and notifies the BlobReader object when the read
+   * operation is finished.
+   *
+   * @param aBlob       The input blob
+   * @param aGlobal     The global object under which the read should happen.
+   * @param aBlobReader The blob reader object to be notified when finished.
+   */
+  static nsresult SlurpBlob(dom::Blob* aBlob, nsIGlobalObject* aGlobal,
+                            BlobReader* aBlobReader);
+
+  /**
+   * OnModifyDocumentInternal() is called by OnModifyDocument().
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT nsresult OnModifyDocumentInternal();
+
+  /**
+   * For saving allocation cost in the constructor of
+   * EditorBase::TopLevelEditSubActionData, we should reuse same RangeItem
+   * instance with all top level edit sub actions.
+   * The instance is always cleared when TopLevelEditSubActionData is
+   * destructed and the class is stack only class so that we don't need
+   * to (and also should not) add the RangeItem into the cycle collection.
+   */
+  [[nodiscard]] inline already_AddRefed<RangeItem>
+  GetSelectedRangeItemForTopLevelEditSubAction() const;
+
+  /**
+   * For saving allocation cost in the constructor of
+   * EditorBase::TopLevelEditSubActionData, we should reuse same nsRange
+   * instance with all top level edit sub actions.
+   * The instance is always cleared when TopLevelEditSubActionData is
+   * destructed, but AbstractRange::mOwner keeps grabbing the owner document
+   * so that we need to make it in the cycle collection.
+   */
+  [[nodiscard]] inline already_AddRefed<nsRange>
+  GetChangedRangeForTopLevelEditSubAction() const;
+
+  MOZ_CAN_RUN_SCRIPT void DidDoTransaction(
+      TransactionManager& aTransactionManager, nsITransaction& aTransaction,
+      nsresult aDoTransactionResult) {
+    if (mComposerCommandsUpdater) {
+      RefPtr<ComposerCommandsUpdater> updater(mComposerCommandsUpdater);
+      updater->DidDoTransaction(aTransactionManager);
+    }
+  }
+
+  MOZ_CAN_RUN_SCRIPT void DidUndoTransaction(
+      TransactionManager& aTransactionManager, nsITransaction& aTransaction,
+      nsresult aUndoTransactionResult) {
+    if (mComposerCommandsUpdater) {
+      RefPtr<ComposerCommandsUpdater> updater(mComposerCommandsUpdater);
+      updater->DidUndoTransaction(aTransactionManager);
+    }
+  }
+
+  MOZ_CAN_RUN_SCRIPT void DidRedoTransaction(
+      TransactionManager& aTransactionManager, nsITransaction& aTransaction,
+      nsresult aRedoTransactionResult) {
+    if (mComposerCommandsUpdater) {
+      RefPtr<ComposerCommandsUpdater> updater(mComposerCommandsUpdater);
+      updater->DidRedoTransaction(aTransactionManager);
+    }
+  }
+
+ protected:
+  /**
+   * IndentListChildWithTransaction() is a helper method of
+   * Handle(CSS|HTML)IndentAtSelectionInternal().
+   *
+   * @param aSubListElement     [in/out] Specify a sub-list element of the
+   *                            container of aPointInListElement or nullptr.
+   *                            When there is no proper sub-list element to
+   *                            move aContentMovingToSubList, this method
+   *                            inserts a new sub-list element and update this
+   *                            to it.
+   * @param aPointInListElement A point in a list element whose child should
+   *                            be indented.  If this method creates new list
+   *                            element into the list element, this inserts
+   *                            the new list element to this point.
+   * @param aContentMovingToSubList
+   *                            A content node which is a child of a list
+   *                            element and should be moved into a sub-list
+   *                            element.
+   * @param aEditingHost        The editing host.
+   * @return                    A candidate caret position.
+   */
+  [[nodiscard]] MOZ_CAN_RUN_SCRIPT Result<EditorDOMPoint, nsresult>
+  IndentListChildWithTransaction(RefPtr<Element>* aSubListElement,
+                                 const EditorDOMPoint& aPointInListElement,
+                                 nsIContent& aContentMovingToSubList,
+                                 const Element& aEditingHost);
+
+  /**
+   * Stack based helper class for saving/restoring selection.  Note that this
+   * assumes that the nodes involved are still around afterwords!
+   */
+  class AutoSelectionRestorer final {
+   public:
+    AutoSelectionRestorer() = delete;
+    explicit AutoSelectionRestorer(const AutoSelectionRestorer& aOther) =
+        delete;
+    AutoSelectionRestorer(AutoSelectionRestorer&& aOther) = delete;
+
+    /**
+     * Constructor responsible for remembering all state needed to restore
+     * aSelection.
+     * XXX This constructor and the destructor should be marked as
+     *     `MOZ_CAN_RUN_SCRIPT`, but it's impossible due to this may be used
+     *     with `Maybe`.
+     */
+    MOZ_CAN_RUN_SCRIPT_BOUNDARY explicit AutoSelectionRestorer(
+        HTMLEditor& aHTMLEditor);
+
+    /**
+     * Destructor restores mSelection to its former state
+     */
+    MOZ_CAN_RUN_SCRIPT_BOUNDARY ~AutoSelectionRestorer();
+
+    /**
+     * Abort() cancels to restore the selection.
+     */
+    void Abort();
+
+    bool MaybeRestoreSelectionLater() const { return !!mHTMLEditor; }
+
+   protected:
+    // The lifetime must be guaranteed by the creator of this instance.
+    MOZ_KNOWN_LIVE HTMLEditor* mHTMLEditor = nullptr;
+  };
+
+  /**
+   * Stack based helper class for calling EditorBase::EndTransactionInternal().
+   * NOTE:  This does not suppress multiple input events.  In most cases,
+   *        only one "input" event should be fired for an edit action rather
+   *        than per edit sub-action.  In such case, you should use
+   *        EditorBase::AutoPlaceholderBatch instead.
+   */
+  class MOZ_RAII AutoTransactionBatch final {
+   public:
+    /**
+     * @param aRequesterFuncName function name which wants to end the batch.
+     * This won't be stored nor exposed to selection listeners etc, used only
+     * for logging. This MUST be alive when the destructor runs.
+     */
+    MOZ_CAN_RUN_SCRIPT explicit AutoTransactionBatch(
+        HTMLEditor& aHTMLEditor, const char* aRequesterFuncName)
+        : mHTMLEditor(aHTMLEditor), mRequesterFuncName(aRequesterFuncName) {
+      MOZ_KnownLive(mHTMLEditor).BeginTransactionInternal(mRequesterFuncName);
+    }
+
+    MOZ_CAN_RUN_SCRIPT ~AutoTransactionBatch() {
+      MOZ_KnownLive(mHTMLEditor).EndTransactionInternal(mRequesterFuncName);
+    }
+
+   protected:
+    // The lifetime must be guaranteed by the creator of this instance.
+    MOZ_KNOWN_LIVE HTMLEditor& mHTMLEditor;
+    const char* const mRequesterFuncName;
+  };
+
+  RefPtr<PendingStyles> mPendingStylesToApplyToNewContent;
+  RefPtr<ComposerCommandsUpdater> mComposerCommandsUpdater;
+
+  // Used by TopLevelEditSubActionData::mSelectedRange.
+  mutable RefPtr<RangeItem> mSelectedRangeForTopLevelEditSubAction;
+  // Used by TopLevelEditSubActionData::mChangedRange.
+  mutable RefPtr<nsRange> mChangedRangeForTopLevelEditSubAction;
+
+  RefPtr<Runnable> mPendingRootElementUpdatedRunner;
+  RefPtr<Runnable> mPendingDocumentModifiedRunner;
+
+  // mPaddingBRElementForEmptyEditor should be used for placing caret
+  // at proper position when editor is empty.
+  RefPtr<dom::HTMLBRElement> mPaddingBRElementForEmptyEditor;
+
+  bool mCRInParagraphCreatesParagraph;
+
+  // resizing
+  bool mIsObjectResizingEnabled;
+  bool mIsResizing;
+  bool mPreserveRatio;
+  bool mResizedObjectIsAnImage;
+
+  // absolute positioning
+  bool mIsAbsolutelyPositioningEnabled;
+  bool mResizedObjectIsAbsolutelyPositioned;
+  bool mGrabberClicked;
+  bool mIsMoving;
+
+  bool mSnapToGridEnabled;
+
+  // inline table editing
+  bool mIsInlineTableEditingEnabled;
+
+  bool mIsCSSPrefChecked;
+
+  // resizing
+  ManualNACPtr mTopLeftHandle;
+  ManualNACPtr mTopHandle;
+  ManualNACPtr mTopRightHandle;
+  ManualNACPtr mLeftHandle;
+  ManualNACPtr mRightHandle;
+  ManualNACPtr mBottomLeftHandle;
+  ManualNACPtr mBottomHandle;
+  ManualNACPtr mBottomRightHandle;
+
+  RefPtr<Element> mActivatedHandle;
+
+  ManualNACPtr mResizingShadow;
+  ManualNACPtr mResizingInfo;
+
+  RefPtr<Element> mResizedObject;
+
+  int32_t mOriginalX;
+  int32_t mOriginalY;
+
+  int32_t mResizedObjectX;
+  int32_t mResizedObjectY;
+  int32_t mResizedObjectWidth;
+  int32_t mResizedObjectHeight;
+
+  int32_t mResizedObjectMarginLeft;
+  int32_t mResizedObjectMarginTop;
+  int32_t mResizedObjectBorderLeft;
+  int32_t mResizedObjectBorderTop;
+
+  int32_t mXIncrementFactor;
+  int32_t mYIncrementFactor;
+  int32_t mWidthIncrementFactor;
+  int32_t mHeightIncrementFactor;
+
+  int8_t mInfoXIncrement;
+  int8_t mInfoYIncrement;
+
+  // absolute positioning
+  int32_t mPositionedObjectX;
+  int32_t mPositionedObjectY;
+  int32_t mPositionedObjectWidth;
+  int32_t mPositionedObjectHeight;
+
+  int32_t mPositionedObjectMarginLeft;
+  int32_t mPositionedObjectMarginTop;
+  int32_t mPositionedObjectBorderLeft;
+  int32_t mPositionedObjectBorderTop;
+
+  RefPtr<Element> mAbsolutelyPositionedObject;
+  ManualNACPtr mGrabber;
+  ManualNACPtr mPositioningShadow;
+
+  int32_t mGridSize;
+
+  // inline table editing
+  RefPtr<Element> mInlineEditedCell;
+
+  ManualNACPtr mAddColumnBeforeButton;
+  ManualNACPtr mRemoveColumnButton;
+  ManualNACPtr mAddColumnAfterButton;
+
+  ManualNACPtr mAddRowBeforeButton;
+  ManualNACPtr mRemoveRowButton;
+  ManualNACPtr mAddRowAfterButton;
+
+  void AddMouseClickListener(Element* aElement);
+  void RemoveMouseClickListener(Element* aElement);
+
+  bool mDisabledLinkHandling = false;
+  bool mOldLinkHandlingEnabled = false;
+
+  bool mHasBeforeInputBeenCanceled = false;
+
+  ParagraphSeparator mDefaultParagraphSeparator;
+
+  friend class AlignStateAtSelection;  // CollectEditableTargetNodes,
+                                       // CollectNonEditableNodes
+  friend class AutoRangeArray;  // RangeUpdaterRef, SplitNodeWithTransaction,
+                                // SplitInlineAncestorsAtRangeBoundaries
+  friend class AutoSelectionSetterAfterTableEdit;  // SetSelectionAfterEdit
+  friend class
+      AutoSetTemporaryAncestorLimiter;  // InitializeSelectionAncestorLimit
+  friend class CSSEditUtils;            // DoTransactionInternal, HasAttributes,
+                                        // RemoveContainerWithTransaction
+  friend class EditorBase;              // ComputeTargetRanges,
+                            // GetChangedRangeForTopLevelEditSubAction,
+                            // GetSelectedRangeItemForTopLevelEditSubAction,
+                            // MaybeCreatePaddingBRElementForEmptyEditor,
+                            // PrepareToInsertBRElement,
+                            // ReflectPaddingBRElementForEmptyEditor,
+                            // RefreshEditingUI,
+                            // RemoveEmptyInclusiveAncestorInlineElements,
+                            // mComposerUpdater, mHasBeforeInputBeenCanceled
+  friend class JoinNodesTransaction;  // DidJoinNodesTransaction, DoJoinNodes,
+                                      // DoSplitNode, // RangeUpdaterRef
+  friend class ListElementSelectionState;      // CollectEditTargetNodes,
+                                               // CollectNonEditableNodes
+  friend class ListItemElementSelectionState;  // CollectEditTargetNodes,
+                                               // CollectNonEditableNodes
+  friend class MoveNodeTransaction;  // AllowsTransactionsToChangeSelection,
+                                     // CollapseSelectionTo, MarkElementDirty,
+                                     // RangeUpdaterRef
+  friend class ParagraphStateAtSelection;  // CollectChildren,
+                                           // CollectEditTargetNodes,
+                                           // CollectListChildren,
+                                           // CollectNonEditableNodes,
+                                           // CollectTableChildren
+  friend class SlurpBlobEventListener;     // BlobReader
+  friend class SplitNodeTransaction;       // DoJoinNodes, DoSplitNode
+  friend class TransactionManager;  // DidDoTransaction, DidRedoTransaction,
+                                    // DidUndoTransaction
+  friend class
+      WhiteSpaceVisibilityKeeper;  // AutoMoveOneLineHandler
+                                   // CanMoveChildren,
+                                   // ChangeListElementType,
+                                   // DeleteNodeWithTransaction,
+                                   // DeleteTextAndTextNodesWithTransaction,
+                                   // JoinNearestEditableNodesWithTransaction,
+                                   // MoveChildrenWithTransaction,
+                                   // SplitAncestorStyledInlineElementsAt,
+                                   // TreatEmptyTextNodes
+};
+
+/**
+ * ListElementSelectionState class gets which list element is selected right
+ * now.
+ */
+class MOZ_STACK_CLASS ListElementSelectionState final {
+ public:
+  ListElementSelectionState() = delete;
+  ListElementSelectionState(HTMLEditor& aHTMLEditor, ErrorResult& aRv);
+
+  bool IsOLElementSelected() const { return mIsOLElementSelected; }
+  bool IsULElementSelected() const { return mIsULElementSelected; }
+  bool IsDLElementSelected() const { return mIsDLElementSelected; }
+  bool IsNotOneTypeListElementSelected() const {
+    return (mIsOLElementSelected + mIsULElementSelected + mIsDLElementSelected +
+            mIsOtherContentSelected) > 1;
+  }
+
+ private:
+  bool mIsOLElementSelected = false;
+  bool mIsULElementSelected = false;
+  bool mIsDLElementSelected = false;
+  bool mIsOtherContentSelected = false;
+};
+
+/**
+ * ListItemElementSelectionState class gets which list item element is selected
+ * right now.
+ */
+class MOZ_STACK_CLASS ListItemElementSelectionState final {
+ public:
+  ListItemElementSelectionState() = delete;
+  ListItemElementSelectionState(HTMLEditor& aHTMLEditor, ErrorResult& aRv);
+
+  bool IsLIElementSelected() const { return mIsLIElementSelected; }
+  bool IsDTElementSelected() const { return mIsDTElementSelected; }
+  bool IsDDElementSelected() const { return mIsDDElementSelected; }
+  bool IsNotOneTypeDefinitionListItemElementSelected() const {
+    return (mIsDTElementSelected + mIsDDElementSelected +
+            mIsOtherElementSelected) > 1;
+  }
+
+ private:
+  bool mIsLIElementSelected = false;
+  bool mIsDTElementSelected = false;
+  bool mIsDDElementSelected = false;
+  bool mIsOtherElementSelected = false;
+};
+
+/**
+ * AlignStateAtSelection class gets alignment at selection.
+ * XXX This currently returns only first alignment.
+ */
+class MOZ_STACK_CLASS AlignStateAtSelection final {
+ public:
+  AlignStateAtSelection() = delete;
+  MOZ_CAN_RUN_SCRIPT AlignStateAtSelection(HTMLEditor& aHTMLEditor,
+                                           ErrorResult& aRv);
+
+  nsIHTMLEditor::EAlignment AlignmentAtSelectionStart() const {
+    return mFirstAlign;
+  }
+  bool IsSelectionRangesFound() const { return mFoundSelectionRanges; }
+
+ private:
+  nsIHTMLEditor::EAlignment mFirstAlign = nsIHTMLEditor::eLeft;
+  bool mFoundSelectionRanges = false;
+};
+
+/**
+ * ParagraphStateAtSelection class gets format block types around selection.
+ */
+class MOZ_STACK_CLASS ParagraphStateAtSelection final {
+ public:
+  using FormatBlockMode = HTMLEditor::FormatBlockMode;
+
+  ParagraphStateAtSelection() = delete;
+  /**
+   * @param aFormatBlockMode    Whether HTML formatBlock command or XUL
+   *                            paragraphState command.
+   */
+  ParagraphStateAtSelection(HTMLEditor& aHTMLEditor,
+                            FormatBlockMode aFormatBlockMode, ErrorResult& aRv);
+
+  /**
+   * GetFirstParagraphStateAtSelection() returns:
+   * - nullptr if there is no format blocks nor inline nodes.
+   * - nsGkAtoms::_empty if first node is not in any format block.
+   * - a tag name of format block at first node.
+   * XXX See the private method explanations.  If selection ranges contains
+   *     non-format block first, it'll be check after its siblings.  Therefore,
+   *     this may return non-first paragraph state.
+   */
+  nsAtom* GetFirstParagraphStateAtSelection() const {
+    return mIsMixed && mIsInDLElement ? nsGkAtoms::dl
+                                      : mFirstParagraphState.get();
+  }
+
+  /**
+   * If selected nodes are not in same format node nor only in no-format blocks,
+   * this returns true.
+   */
+  bool IsMixed() const { return mIsMixed && !mIsInDLElement; }
+
+ private:
+  using EditorType = EditorBase::EditorType;
+
+  [[nodiscard]] static bool IsFormatElement(FormatBlockMode aFormatBlockMode,
+                                            const nsIContent& aContent);
+
+  /**
+   * AppendDescendantFormatNodesAndFirstInlineNode() appends descendant
+   * format blocks and first inline child node in aNonFormatBlockElement to
+   * the last of the array (not inserting where aNonFormatBlockElement is,
+   * so that the node order becomes randomly).
+   *
+   * @param aArrayOfContents            [in/out] Found descendant format blocks
+   *                                    and first inline node in each non-format
+   *                                    block will be appended to this.
+   * @param aFormatBlockMode            Whether HTML formatBlock command or XUL
+   *                                    paragraphState command.
+   * @param aNonFormatBlockElement      Must be a non-format block element.
+   */
+  static void AppendDescendantFormatNodesAndFirstInlineNode(
+      nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents,
+      FormatBlockMode aFormatBlockMode, dom::Element& aNonFormatBlockElement);
+
+  /**
+   * CollectEditableFormatNodesInSelection() collects only editable nodes
+   * around selection ranges (with `AutoRangeArray::ExtendRangesToWrapLines()`
+   * and `HTMLEditor::CollectEditTargetNodes()`, see its document for the
+   * detail). If it includes list, list item or table related elements, they
+   * will be replaced their children.
+   *
+   * @param aFormatBlockMode            Whether HTML formatBlock command or XUL
+   *                                    paragraphState command.
+   */
+  static nsresult CollectEditableFormatNodesInSelection(
+      HTMLEditor& aHTMLEditor, FormatBlockMode aFormatBlockMode,
+      const dom::Element& aEditingHost,
+      nsTArray<OwningNonNull<nsIContent>>& aArrayOfContents);
+
+  RefPtr<nsAtom> mFirstParagraphState;
+  bool mIsInDLElement = false;
+  bool mIsMixed = false;
+};
+
+}  // namespace mozilla
+
+mozilla::HTMLEditor* nsIEditor::AsHTMLEditor() {
+  MOZ_DIAGNOSTIC_ASSERT(IsHTMLEditor());
+  return static_cast<mozilla::HTMLEditor*>(this);
+}
+
+const mozilla::HTMLEditor* nsIEditor::AsHTMLEditor() const {
+  MOZ_DIAGNOSTIC_ASSERT(IsHTMLEditor());
+  return static_cast<const mozilla::HTMLEditor*>(this);
+}
+
+mozilla::HTMLEditor* nsIEditor::GetAsHTMLEditor() {
+  return AsEditorBase()->IsHTMLEditor() ? AsHTMLEditor() : nullptr;
+}
+
+const mozilla::HTMLEditor* nsIEditor::GetAsHTMLEditor() const {
+  return AsEditorBase()->IsHTMLEditor() ? AsHTMLEditor() : nullptr;
+}
+
+#endif  // #ifndef mozilla_HTMLEditor_h