diff --git a/dom/base/ContentIterator.h b/dom/base/ContentIterator.h
index 67962d41d54..5a648de38c9 100644
--- a/dom/base/ContentIterator.h
+++ b/dom/base/ContentIterator.h
@@ -39,13 +39,15 @@ class ContentIteratorBase {
    * (https://dom.spec.whatwg.org/#concept-tree-inclusive-descendant) of
    * aRoot.
    */
-  virtual nsresult Init(nsINode* aRoot);
+  [[nodiscard]] virtual nsresult Init(nsINode* aRoot);
 
-  virtual nsresult Init(dom::AbstractRange* aRange);
-  virtual nsresult Init(nsINode* aStartContainer, uint32_t aStartOffset,
-                        nsINode* aEndContainer, uint32_t aEndOffset);
-  virtual nsresult Init(const RawRangeBoundary& aStart,
-                        const RawRangeBoundary& aEnd);
+  [[nodiscard]] virtual nsresult Init(dom::AbstractRange* aRange);
+  [[nodiscard]] virtual nsresult Init(nsINode* aStartContainer,
+                                      uint32_t aStartOffset,
+                                      nsINode* aEndContainer,
+                                      uint32_t aEndOffset);
+  [[nodiscard]] virtual nsresult Init(const RawRangeBoundary& aStart,
+                                      const RawRangeBoundary& aEnd);
 
   virtual void First();
   virtual void Last();
@@ -56,7 +58,7 @@ class ContentIteratorBase {
 
   bool IsDone() const { return !mCurNode; }
 
-  virtual nsresult PositionAt(nsINode* aCurNode);
+  [[nodiscard]] virtual nsresult PositionAt(nsINode* aCurNode);
 
  protected:
   enum class Order {
@@ -76,8 +78,8 @@ class ContentIteratorBase {
    * - aStartOffset and aEndOffset are valid for its container.
    * - The start point and the end point are in document order.
    */
-  nsresult InitInternal(const RawRangeBoundary& aStart,
-                        const RawRangeBoundary& aEnd);
+  [[nodiscard]] nsresult InitInternal(const RawRangeBoundary& aStart,
+                                      const RawRangeBoundary& aEnd);
 
   // Recursively get the deepest first/last child of aRoot.  This will return
   // aRoot itself if it has no children.
@@ -108,11 +110,11 @@ class ContentIteratorBase {
 
   void SetEmpty();
 
-  NodeType mCurNode;
-  NodeType mFirst;
-  NodeType mLast;
+  NodeType mCurNode = nullptr;
+  NodeType mFirst = nullptr;
+  NodeType mLast = nullptr;
   // See <https://dom.spec.whatwg.org/#concept-tree-inclusive-ancestor>.
-  NodeType mClosestCommonInclusiveAncestor;
+  NodeType mClosestCommonInclusiveAncestor = nullptr;
 
   Maybe<nsMutationGuard> mMutationGuard;
   Maybe<JS::AutoAssertNoGC> mAssertNoGC;
@@ -227,9 +229,9 @@ class ContentSubtreeIterator final : public SafeContentIteratorBase {
   /**
    * Not supported.
    */
-  virtual nsresult Init(nsINode* aRoot) override;
+  [[nodiscard]] virtual nsresult Init(nsINode* aRoot) override;
 
-  virtual nsresult Init(dom::AbstractRange* aRange) override;
+  [[nodiscard]] virtual nsresult Init(dom::AbstractRange* aRange) override;
 
   /**
    * Initialize the iterator with aRange that does correct things
@@ -252,11 +254,15 @@ class ContentSubtreeIterator final : public SafeContentIteratorBase {
    * Examples of what nodes will be returned can be found
    * at test_content_iterator_subtree_shadow_tree.html.
    */
-  nsresult InitWithAllowCrossShadowBoundary(dom::AbstractRange* aRange);
-  virtual nsresult Init(nsINode* aStartContainer, uint32_t aStartOffset,
-                        nsINode* aEndContainer, uint32_t aEndOffset) override;
-  virtual nsresult Init(const RawRangeBoundary& aStartBoundary,
-                        const RawRangeBoundary& aEndBoundary) override;
+  [[nodiscard]] nsresult InitWithAllowCrossShadowBoundary(
+      dom::AbstractRange* aRange);
+  [[nodiscard]] virtual nsresult Init(nsINode* aStartContainer,
+                                      uint32_t aStartOffset,
+                                      nsINode* aEndContainer,
+                                      uint32_t aEndOffset) override;
+  [[nodiscard]] virtual nsresult Init(
+      const RawRangeBoundary& aStartBoundary,
+      const RawRangeBoundary& aEndBoundary) override;
 
   void Next() override;
   void Prev() override;
@@ -265,7 +271,7 @@ class ContentSubtreeIterator final : public SafeContentIteratorBase {
   // Must override these because we don't do PositionAt
   void Last() override;
 
-  nsresult PositionAt(nsINode* aCurNode) override;
+  [[nodiscard]] nsresult PositionAt(nsINode* aCurNode) override;
 
   friend void ImplCycleCollectionTraverse(nsCycleCollectionTraversalCallback&,
                                           ContentSubtreeIterator&, const char*,
@@ -301,7 +307,7 @@ class ContentSubtreeIterator final : public SafeContentIteratorBase {
   /**
    * Callers must guarantee that mRange isn't nullptr and is positioned.
    */
-  nsresult InitWithRange();
+  [[nodiscard]] nsresult InitWithRange();
 
   // Returns the highest inclusive ancestor of aNode that's in the range
   // (possibly aNode itself).  Returns null if aNode is null, or is not itself
diff --git a/dom/base/Selection.cpp b/dom/base/Selection.cpp
index ff2cff76730..ab3f8ec7941 100644
--- a/dom/base/Selection.cpp
+++ b/dom/base/Selection.cpp
@@ -346,7 +346,10 @@ const nsTHashSet<const nsINode*>& SelectionNodeCache::MaybeCollect(
       const nsINode* endContainer =
           endRef.IsEndOfContainer() ? nullptr : endRef.Container();
       UnsafePreContentIterator iter;
-      iter.Init(range);
+      nsresult rv = iter.Init(range);
+      if (NS_FAILED(rv)) {
+        continue;
+      }
       for (; !iter.IsDone(); iter.Next()) {
         if (const nsINode* node = iter.GetCurrentNode()) {
           // Only collect start and end container if they are fully
@@ -1944,7 +1947,10 @@ nsresult Selection::SelectFrames(nsPresContext* aPresContext,
   }
 
   ContentSubtreeIterator subtreeIter;
-  subtreeIter.InitWithAllowCrossShadowBoundary(&aRange);
+  nsresult rv = subtreeIter.InitWithAllowCrossShadowBoundary(&aRange);
+  if (NS_FAILED(rv)) {
+    return rv;
+  }
   if (isFirstContentTextNode && !subtreeIter.IsDone() &&
       subtreeIter.GetCurrentNode() == startContent) {
     subtreeIter.Next();  // first content has already been handled.
diff --git a/editor/libeditor/HTMLEditor.cpp b/editor/libeditor/HTMLEditor.cpp
index bb025bbb783..89093f579d7 100644
--- a/editor/libeditor/HTMLEditor.cpp
+++ b/editor/libeditor/HTMLEditor.cpp
@@ -3539,7 +3539,11 @@ already_AddRefed<Element> HTMLEditor::GetSelectedElement(const nsAtom* aTagName,
   }
 
   PostContentIterator postOrderIter;
-  postOrderIter.Init(firstRange);
+  nsresult rv = postOrderIter.Init(firstRange);
+  if (NS_FAILED(rv)) {
+    aRv.Throw(rv);
+    return nullptr;
+  }
 
   RefPtr<Element> lastElementInRange;
   for (nsINode* lastNodeInRange = nullptr; !postOrderIter.IsDone();