diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index b136d30e65ee..6171bf01e4f1 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -990,8 +990,8 @@ void nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor) {
           // dispatching event to Window object in a content page and
           // propagating the event to a chrome Element.
           if (targetInKnownToBeHandledScope &&
-              nsContentUtils::ContentIsShadowIncludingDescendantOf(
-                  this, targetInKnownToBeHandledScope->SubtreeRoot())) {
+              IsShadowIncludingInclusiveDescendantOf(
+                  targetInKnownToBeHandledScope->SubtreeRoot())) {
             // Part of step 11.4.
             // "If target's root is a shadow-including inclusive ancestor of
             //  parent, then"
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 1e72f01da338..e791ed474704 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -2200,32 +2200,6 @@ bool nsContentUtils::ContentIsHostIncludingDescendantOf(
   return false;
 }
 
-bool nsContentUtils::ContentIsShadowIncludingDescendantOf(
-    const nsINode* aPossibleDescendant, const nsINode* aPossibleAncestor) {
-  MOZ_ASSERT(aPossibleDescendant, "The possible descendant is null!");
-  MOZ_ASSERT(aPossibleAncestor, "The possible ancestor is null!");
-
-  if (aPossibleAncestor == aPossibleDescendant->GetComposedDoc()) {
-    return true;
-  }
-
-  do {
-    if (aPossibleDescendant == aPossibleAncestor) {
-      return true;
-    }
-
-    if (aPossibleDescendant->NodeType() == nsINode::DOCUMENT_FRAGMENT_NODE) {
-      ShadowRoot* shadowRoot =
-          ShadowRoot::FromNode(const_cast<nsINode*>(aPossibleDescendant));
-      aPossibleDescendant = shadowRoot ? shadowRoot->GetHost() : nullptr;
-    } else {
-      aPossibleDescendant = aPossibleDescendant->GetParentNode();
-    }
-  } while (aPossibleDescendant);
-
-  return false;
-}
-
 // static
 bool nsContentUtils::ContentIsCrossDocDescendantOf(nsINode* aPossibleDescendant,
                                                    nsINode* aPossibleAncestor) {
@@ -2285,7 +2259,7 @@ nsINode* nsContentUtils::Retarget(nsINode* aTargetA, nsINode* aTargetB) {
     }
 
     // or A's root is a shadow-including inclusive ancestor of B...
-    if (nsContentUtils::ContentIsShadowIncludingDescendantOf(aTargetB, root)) {
+    if (aTargetB->IsShadowIncludingInclusiveDescendantOf(root)) {
       // ...then return A.
       return aTargetA;
     }
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 7162a83ec429..3664849ece7a 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -344,13 +344,6 @@ class nsContentUtils {
   static bool ContentIsHostIncludingDescendantOf(
       const nsINode* aPossibleDescendant, const nsINode* aPossibleAncestor);
 
-  /**
-   * Similar to above, but does special case only ShadowRoot,
-   * not HTMLTemplateElement.
-   */
-  static bool ContentIsShadowIncludingDescendantOf(
-      const nsINode* aPossibleDescendant, const nsINode* aPossibleAncestor);
-
   /**
    * Similar to nsINode::IsInclusiveDescendantOf except it crosses document
    * boundaries, this function uses ancestor/descendant relations in the
diff --git a/dom/base/nsINode.cpp b/dom/base/nsINode.cpp
index d3165c847968..d70f288a977a 100644
--- a/dom/base/nsINode.cpp
+++ b/dom/base/nsINode.cpp
@@ -131,6 +131,26 @@ bool nsINode::IsInclusiveDescendantOf(const nsINode* aNode) const {
   return false;
 }
 
+bool nsINode::IsShadowIncludingInclusiveDescendantOf(
+    const nsINode* aNode) const {
+  MOZ_ASSERT(aNode, "The node is nullptr.");
+
+  if (this->GetComposedDoc() == aNode) {
+    return true;
+  }
+
+  const nsINode* node = this;
+  do {
+    if (node == aNode) {
+      return true;
+    }
+
+    node = node->GetParentOrShadowHostNode();
+  } while (node);
+
+  return false;
+}
+
 nsINode::nsSlots::nsSlots() : mWeakReference(nullptr) {}
 
 nsINode::nsSlots::~nsSlots() {
diff --git a/dom/base/nsINode.h b/dom/base/nsINode.h
index 609cc3bea76c..3c15c375e795 100644
--- a/dom/base/nsINode.h
+++ b/dom/base/nsINode.h
@@ -424,6 +424,13 @@ class nsINode : public mozilla::dom::EventTarget {
    */
   bool IsInclusiveDescendantOf(const nsINode* aNode) const;
 
+  /**
+   * https://dom.spec.whatwg.org/#concept-shadow-including-inclusive-descendant
+   *
+   * @param aNode must not be nullptr.
+   */
+  bool IsShadowIncludingInclusiveDescendantOf(const nsINode* aNode) const;
+
   /**
    * Return this node as a document fragment. Asserts IsDocumentFragment().
    *
diff --git a/extensions/spellcheck/src/mozInlineSpellChecker.cpp b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
index de5ce5e6dc59..91985115020b 100644
--- a/extensions/spellcheck/src/mozInlineSpellChecker.cpp
+++ b/extensions/spellcheck/src/mozInlineSpellChecker.cpp
@@ -222,8 +222,7 @@ nsresult mozInlineSpellStatus::InitForNavigation(
   }
   // the anchor node might not be in the DOM anymore, check
   if (root && aOldAnchorNode &&
-      !nsContentUtils::ContentIsShadowIncludingDescendantOf(aOldAnchorNode,
-                                                            root)) {
+      !aOldAnchorNode->IsShadowIncludingInclusiveDescendantOf(root)) {
     *aContinue = false;
     return NS_OK;
   }
@@ -1271,10 +1270,8 @@ nsresult mozInlineSpellChecker::DoSpellCheck(
     // aWordUtil.GetRootNode()
     nsINode* rootNode = aWordUtil.GetRootNode();
     if (!beginNode->IsInComposedDoc() || !endNode->IsInComposedDoc() ||
-        !nsContentUtils::ContentIsShadowIncludingDescendantOf(beginNode,
-                                                              rootNode) ||
-        !nsContentUtils::ContentIsShadowIncludingDescendantOf(endNode,
-                                                              rootNode)) {
+        !beginNode->IsShadowIncludingInclusiveDescendantOf(rootNode) ||
+        !endNode->IsShadowIncludingInclusiveDescendantOf(rootNode)) {
       // Just bail out and don't try to spell-check this
       return NS_OK;
     }