diff --git a/dom/base/Document.h b/dom/base/Document.h
index 8cd8269c449d..6c779db66834 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -606,7 +606,7 @@ class Document : public nsINode,
                                                     aFocusedRadio, aRadioOut);
   }
   void AddToRadioGroup(const nsAString& aName, HTMLInputElement* aRadio) final {
-    DocumentOrShadowRoot::AddToRadioGroup(aName, aRadio, nullptr);
+    DocumentOrShadowRoot::AddToRadioGroup(aName, aRadio);
   }
   void RemoveFromRadioGroup(const nsAString& aName,
                             HTMLInputElement* aRadio) final {
diff --git a/dom/base/RadioGroupManager.cpp b/dom/base/RadioGroupManager.cpp
index 759b58db0ea4..e7c8a730fae6 100644
--- a/dom/base/RadioGroupManager.cpp
+++ b/dom/base/RadioGroupManager.cpp
@@ -115,11 +115,9 @@ nsresult RadioGroupManager::GetNextRadioButton(const nsAString& aName,
 }
 
 void RadioGroupManager::AddToRadioGroup(const nsAString& aName,
-                                        HTMLInputElement* aRadio,
-                                        nsIContent* aAncestor) {
+                                        HTMLInputElement* aRadio) {
   nsRadioGroupStruct* radioGroup = GetOrCreateRadioGroup(aName);
-  nsContentUtils::AddElementToListByTreeOrder(radioGroup->mRadioButtons, aRadio,
-                                              aAncestor);
+  radioGroup->mRadioButtons.AppendElement(aRadio);
 
   if (aRadio->IsRequired()) {
     radioGroup->mRequiredRadioCount++;
diff --git a/dom/base/RadioGroupManager.h b/dom/base/RadioGroupManager.h
index 1119462a7d98..f25bf5e7539d 100644
--- a/dom/base/RadioGroupManager.h
+++ b/dom/base/RadioGroupManager.h
@@ -12,8 +12,6 @@
 #include "nsIRadioGroupContainer.h"
 #include "nsClassHashtable.h"
 
-class nsIContent;
-
 namespace mozilla {
 
 namespace html {
@@ -39,8 +37,7 @@ class RadioGroupManager {
   nsresult GetNextRadioButton(const nsAString& aName, const bool aPrevious,
                               HTMLInputElement* aFocusedRadio,
                               HTMLInputElement** aRadioOut);
-  void AddToRadioGroup(const nsAString& aName, HTMLInputElement* aRadio,
-                       nsIContent* aAncestor);
+  void AddToRadioGroup(const nsAString& aName, HTMLInputElement* aRadio);
   void RemoveFromRadioGroup(const nsAString& aName, HTMLInputElement* aRadio);
   uint32_t GetRequiredRadioCount(const nsAString& aName) const;
   void RadioRequiredWillChange(const nsAString& aName, bool aRequiredAdded);
diff --git a/dom/base/ShadowRoot.h b/dom/base/ShadowRoot.h
index aceb3ab56a73..d0678e3e3df7 100644
--- a/dom/base/ShadowRoot.h
+++ b/dom/base/ShadowRoot.h
@@ -255,7 +255,7 @@ class ShadowRoot final : public DocumentFragment,
   }
   void AddToRadioGroup(const nsAString& aName,
                        HTMLInputElement* aRadio) override {
-    DocumentOrShadowRoot::AddToRadioGroup(aName, aRadio, this);
+    DocumentOrShadowRoot::AddToRadioGroup(aName, aRadio);
   }
   void RemoveFromRadioGroup(const nsAString& aName,
                             HTMLInputElement* aRadio) override {
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 7292d41ed4d3..86b69470a294 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -175,7 +175,6 @@
 #include "mozilla/dom/FromParser.h"
 #include "mozilla/dom/HTMLElement.h"
 #include "mozilla/dom/HTMLFormElement.h"
-#include "mozilla/dom/HTMLImageElement.h"
 #include "mozilla/dom/HTMLInputElement.h"
 #include "mozilla/dom/HTMLTextAreaElement.h"
 #include "mozilla/dom/IPCBlob.h"
@@ -11226,84 +11225,6 @@ nsIContent* nsContentUtils::GetClosestLinkInFlatTree(nsIContent* aContent) {
   return nullptr;
 }
 
-namespace {
-
-struct TreePositionComparator {
-  Element* const mChild;
-  nsIContent* const mAncestor;
-  TreePositionComparator(Element* aChild, nsIContent* aAncestor)
-      : mChild(aChild), mAncestor(aAncestor) {}
-  int operator()(Element* aElement) const {
-    return nsLayoutUtils::CompareTreePosition(mChild, aElement, mAncestor);
-  }
-};
-
-}  // namespace
-
-/* static */
-int32_t nsContentUtils::CompareTreePosition(nsIContent* aContent1,
-                                            nsIContent* aContent2,
-                                            const nsIContent* aCommonAncestor) {
-  NS_ASSERTION(aContent1 != aContent2, "Comparing content to itself");
-
-  // TODO: remove the prevent asserts fix, see bug 598468.
-#ifdef DEBUG
-  nsLayoutUtils::gPreventAssertInCompareTreePosition = true;
-  int32_t rVal =
-      nsLayoutUtils::CompareTreePosition(aContent1, aContent2, aCommonAncestor);
-  nsLayoutUtils::gPreventAssertInCompareTreePosition = false;
-
-  return rVal;
-#else   // DEBUG
-  return nsLayoutUtils::CompareTreePosition(aContent1, aContent2,
-                                            aCommonAncestor);
-#endif  // DEBUG
-}
-
-/* static */
-template <typename ElementType, typename ElementPtr>
-bool nsContentUtils::AddElementToListByTreeOrder(nsTArray<ElementType>& aList,
-                                                 ElementPtr aChild,
-                                                 nsIContent* aCommonAncestor) {
-  NS_ASSERTION(aList.IndexOf(aChild) == aList.NoIndex,
-               "aChild already in aList");
-
-  const uint32_t count = aList.Length();
-  ElementType element;
-
-  // Optimize most common case where we insert at the end.
-  int32_t position = -1;
-  if (count > 0) {
-    element = aList[count - 1];
-    position = CompareTreePosition(aChild, element, aCommonAncestor);
-  }
-
-  // If this item comes after the last element, or the elements array is
-  // empty, we append to the end. Otherwise, we do a binary search to
-  // determine where the element should go.
-  if (position >= 0 || count == 0) {
-    aList.AppendElement(aChild);
-    return true;
-  }
-
-  size_t idx;
-  BinarySearchIf(aList, 0, count,
-                 TreePositionComparator(aChild, aCommonAncestor), &idx);
-
-  aList.InsertElementAt(idx, aChild);
-  return false;
-}
-
-template bool nsContentUtils::AddElementToListByTreeOrder(
-    nsTArray<nsGenericHTMLFormElement*>& aList,
-    nsGenericHTMLFormElement* aChild, nsIContent* aAncestor);
-template bool nsContentUtils::AddElementToListByTreeOrder(
-    nsTArray<HTMLImageElement*>& aList, HTMLImageElement* aChild,
-    nsIContent* aAncestor);
-template bool nsContentUtils::AddElementToListByTreeOrder(
-    nsTArray<RefPtr<HTMLInputElement>>& aList, HTMLInputElement* aChild,
-    nsIContent* aAncestor);
-
 namespace mozilla {
 std::ostream& operator<<(std::ostream& aOut,
                          const PreventDefaultResult aPreventDefaultResult) {
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 65f9a8c30018..2f3d5d81ef2c 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -3435,33 +3435,6 @@ class nsContentUtils {
 
   static bool IsExternalProtocol(nsIURI* aURI);
 
-  /**
-   * Add an element to a list, keeping the list sorted by tree order.
-   * Can take a potential ancestor of the elements in order to speed up
-   * tree-order comparisons, if such an ancestor exists.
-   * Returns true if the element is appended to the end of the list.
-   */
-  template <typename ElementType, typename ElementPtr>
-  static bool AddElementToListByTreeOrder(nsTArray<ElementType>& aList,
-                                          ElementPtr aChild,
-                                          nsIContent* aCommonAncestor);
-
-  /**
-   * Compares the position of aContent1 and aContent2 in the document
-   * @param aContent1 First content to compare.
-   * @param aContent2 Second content to compare.
-   * @param aCommonAncestor Potential ancestor of the contents, if one exists.
-   *                        This is only a hint; if it's not an ancestor of
-   *                        aContent1 or aContent2, this function will still
-   *                        work, but it will be slower than normal.
-   * @return < 0 if aContent1 is before aContent2,
-   *         > 0 if aContent1 is after aContent2,
-   *         0 otherwise
-   */
-  static int32_t CompareTreePosition(nsIContent* aContent1,
-                                     nsIContent* aContent2,
-                                     const nsIContent* aCommonAncestor);
-
  private:
   static bool InitializeEventTable();
 
diff --git a/dom/html/HTMLFormControlsCollection.cpp b/dom/html/HTMLFormControlsCollection.cpp
index 406a00a1ea7c..bf25882e05c4 100644
--- a/dom/html/HTMLFormControlsCollection.cpp
+++ b/dom/html/HTMLFormControlsCollection.cpp
@@ -222,9 +222,9 @@ nsresult HTMLFormControlsCollection::GetSortedControls(
     // Determine which of the two elements should be ordered
     // first and add it to the end of the list.
     nsGenericHTMLFormElement* elementToAdd;
-    if (nsContentUtils::CompareTreePosition(mElements[elementsIdx],
-                                            mNotInElements[notInElementsIdx],
-                                            mForm) < 0) {
+    if (HTMLFormElement::CompareFormControlPosition(
+            mElements[elementsIdx], mNotInElements[notInElementsIdx], mForm) <
+        0) {
       elementToAdd = mElements[elementsIdx];
       ++elementsIdx;
     } else {
diff --git a/dom/html/HTMLFormElement.cpp b/dom/html/HTMLFormElement.cpp
index 560ee5ff0708..e30f1e58161e 100644
--- a/dom/html/HTMLFormElement.cpp
+++ b/dom/html/HTMLFormElement.cpp
@@ -1086,6 +1086,44 @@ Element* HTMLFormElement::IndexedGetter(uint32_t aIndex, bool& aFound) {
   return element;
 }
 
+/**
+ * Compares the position of aControl1 and aControl2 in the document
+ * @param aControl1 First control to compare.
+ * @param aControl2 Second control to compare.
+ * @param aForm Parent form of the controls.
+ * @return < 0 if aControl1 is before aControl2,
+ *         > 0 if aControl1 is after aControl2,
+ *         0 otherwise
+ */
+/* static */
+int32_t HTMLFormElement::CompareFormControlPosition(Element* aElement1,
+                                                    Element* aElement2,
+                                                    const nsIContent* aForm) {
+  NS_ASSERTION(aElement1 != aElement2, "Comparing a form control to itself");
+
+  // If an element has a @form, we can assume it *might* be able to not have
+  // a parent and still be in the form.
+  NS_ASSERTION(
+      (aElement1->HasAttr(nsGkAtoms::form) || aElement1->GetParent()) &&
+          (aElement2->HasAttr(nsGkAtoms::form) || aElement2->GetParent()),
+      "Form controls should always have parents");
+
+  // If we pass aForm, we are assuming both controls are form descendants which
+  // is not always the case. This function should work but maybe slower.
+  // However, checking if both elements are form descendants may be slow too...
+  // TODO: remove the prevent asserts fix, see bug 598468.
+#ifdef DEBUG
+  nsLayoutUtils::gPreventAssertInCompareTreePosition = true;
+  int32_t rVal =
+      nsLayoutUtils::CompareTreePosition(aElement1, aElement2, aForm);
+  nsLayoutUtils::gPreventAssertInCompareTreePosition = false;
+
+  return rVal;
+#else   // DEBUG
+  return nsLayoutUtils::CompareTreePosition(aElement1, aElement2, aForm);
+#endif  // DEBUG
+}
+
 #ifdef DEBUG
 /**
  * Checks that all form elements are in document order. Asserts if any pair of
@@ -1168,6 +1206,58 @@ void HTMLFormElement::PostPossibleUsernameEvent() {
   mFormPossibleUsernameEventDispatcher->PostDOMEvent();
 }
 
+namespace {
+
+struct FormComparator {
+  Element* const mChild;
+  HTMLFormElement* const mForm;
+  FormComparator(Element* aChild, HTMLFormElement* aForm)
+      : mChild(aChild), mForm(aForm) {}
+  int operator()(Element* aElement) const {
+    return HTMLFormElement::CompareFormControlPosition(mChild, aElement, mForm);
+  }
+};
+
+}  // namespace
+
+// This function return true if the element, once appended, is the last one in
+// the array.
+template <typename ElementType>
+static bool AddElementToList(nsTArray<ElementType*>& aList, ElementType* aChild,
+                             HTMLFormElement* aForm) {
+  NS_ASSERTION(aList.IndexOf(aChild) == aList.NoIndex,
+               "aChild already in aList");
+
+  const uint32_t count = aList.Length();
+  ElementType* element;
+  bool lastElement = false;
+
+  // Optimize most common case where we insert at the end.
+  int32_t position = -1;
+  if (count > 0) {
+    element = aList[count - 1];
+    position =
+        HTMLFormElement::CompareFormControlPosition(aChild, element, aForm);
+  }
+
+  // If this item comes after the last element, or the elements array is
+  // empty, we append to the end. Otherwise, we do a binary search to
+  // determine where the element should go.
+  if (position >= 0 || count == 0) {
+    // WEAK - don't addref
+    aList.AppendElement(aChild);
+    lastElement = true;
+  } else {
+    size_t idx;
+    BinarySearchIf(aList, 0, count, FormComparator(aChild, aForm), &idx);
+
+    // WEAK - don't addref
+    aList.InsertElementAt(idx, aChild);
+  }
+
+  return lastElement;
+}
+
 nsresult HTMLFormElement::AddElement(nsGenericHTMLFormElement* aChild,
                                      bool aUpdateValidity, bool aNotify) {
   // If an element has a @form, we can assume it *might* be able to not have
@@ -1182,8 +1272,7 @@ nsresult HTMLFormElement::AddElement(nsGenericHTMLFormElement* aChild,
   nsTArray<nsGenericHTMLFormElement*>& controlList =
       childInElements ? mControls->mElements : mControls->mNotInElements;
 
-  bool lastElement =
-      nsContentUtils::AddElementToListByTreeOrder(controlList, aChild, this);
+  bool lastElement = AddElementToList(controlList, aChild, this);
 
 #ifdef DEBUG
   AssertDocumentOrder(controlList, this);
@@ -1219,16 +1308,16 @@ nsresult HTMLFormElement::AddElement(nsGenericHTMLFormElement* aChild,
     // what's in the slot or the child is earlier than the default submit.
     nsGenericHTMLFormElement* oldDefaultSubmit = mDefaultSubmitElement;
     if (!*firstSubmitSlot ||
-        (!lastElement && nsContentUtils::CompareTreePosition(
-                             aChild, *firstSubmitSlot, this) < 0)) {
+        (!lastElement &&
+         CompareFormControlPosition(aChild, *firstSubmitSlot, this) < 0)) {
       // Update mDefaultSubmitElement if it's currently in a valid state.
       // Valid state means either non-null or null because there are in fact
       // no submit elements around.
       if ((mDefaultSubmitElement ||
            (!mFirstSubmitInElements && !mFirstSubmitNotInElements)) &&
           (*firstSubmitSlot == mDefaultSubmitElement ||
-           nsContentUtils::CompareTreePosition(aChild, mDefaultSubmitElement,
-                                               this) < 0)) {
+           CompareFormControlPosition(aChild, mDefaultSubmitElement, this) <
+               0)) {
         mDefaultSubmitElement = aChild;
       }
       *firstSubmitSlot = aChild;
@@ -1359,8 +1448,8 @@ void HTMLFormElement::HandleDefaultSubmitRemoval() {
                  "How did that happen?");
     // Have both; use the earlier one
     mDefaultSubmitElement =
-        nsContentUtils::CompareTreePosition(mFirstSubmitInElements,
-                                            mFirstSubmitNotInElements, this) < 0
+        CompareFormControlPosition(mFirstSubmitInElements,
+                                   mFirstSubmitNotInElements, this) < 0
             ? mFirstSubmitInElements
             : mFirstSubmitNotInElements;
   }
@@ -1708,8 +1797,8 @@ bool HTMLFormElement::IsDefaultSubmitElement(
 
   // We have both kinds of submits.  Check which comes first.
   nsGenericHTMLFormElement* defaultSubmit =
-      nsContentUtils::CompareTreePosition(mFirstSubmitInElements,
-                                          mFirstSubmitNotInElements, this) < 0
+      CompareFormControlPosition(mFirstSubmitInElements,
+                                 mFirstSubmitNotInElements, this) < 0
           ? mFirstSubmitInElements
           : mFirstSubmitNotInElements;
   return aElement == defaultSubmit;
@@ -1922,7 +2011,7 @@ HTMLFormElement::WalkRadioGroup(const nsAString& aName,
 
 void HTMLFormElement::AddToRadioGroup(const nsAString& aName,
                                       HTMLInputElement* aRadio) {
-  RadioGroupManager::AddToRadioGroup(aName, aRadio, this);
+  RadioGroupManager::AddToRadioGroup(aName, aRadio);
 }
 
 void HTMLFormElement::RemoveFromRadioGroup(const nsAString& aName,
@@ -2078,7 +2167,7 @@ nsresult HTMLFormElement::AddElementToTableInternal(
 }
 
 nsresult HTMLFormElement::AddImageElement(HTMLImageElement* aChild) {
-  nsContentUtils::AddElementToListByTreeOrder(mImageElements, aChild, this);
+  AddElementToList(mImageElements, aChild, this);
   return NS_OK;
 }
 
diff --git a/dom/html/HTMLFormElement.h b/dom/html/HTMLFormElement.h
index 6491b8510ac6..80b40c6bcee0 100644
--- a/dom/html/HTMLFormElement.h
+++ b/dom/html/HTMLFormElement.h
@@ -379,6 +379,9 @@ class HTMLFormElement final : public nsGenericHTMLElement,
 
   void GetSupportedNames(nsTArray<nsString>& aRetval);
 
+  static int32_t CompareFormControlPosition(Element* aElement1,
+                                            Element* aElement2,
+                                            const nsIContent* aForm);
 #ifdef DEBUG
   static void AssertDocumentOrder(
       const nsTArray<nsGenericHTMLFormElement*>& aControls, nsIContent* aForm);
diff --git a/dom/html/test/forms/mochitest.ini b/dom/html/test/forms/mochitest.ini
index 111d34f2cbea..2d84628b5f49 100644
--- a/dom/html/test/forms/mochitest.ini
+++ b/dom/html/test/forms/mochitest.ini
@@ -14,7 +14,6 @@ prefs =
 [test_bug1286509.html]
 [test_button_attributes_reflection.html]
 [test_input_radio_indeterminate.html]
-[test_input_radio_navigation_order.html]
 [test_input_radio_radiogroup.html]
 [test_input_radio_required.html]
 [test_change_event.html]
diff --git a/testing/web-platform/tests/html/semantics/forms/the-input-element/radio-keyboard-navigation-order.html b/testing/web-platform/tests/html/semantics/forms/the-input-element/radio-keyboard-navigation-order.html
deleted file mode 100644
index d019ca982c8d..000000000000
--- a/testing/web-platform/tests/html/semantics/forms/the-input-element/radio-keyboard-navigation-order.html
+++ /dev/null
@@ -1,70 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-<meta charset="utf-8">
-<title>Radio button group keyboard navigation order</title>
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
-<script src="/resources/testdriver.js"></script>
-<script src="/resources/testdriver-actions.js"></script>
-<script src="/resources/testdriver-vendor.js"></script>
-</head>
-<body>
-<form id="inside">
-  <input type="radio" name="inside" id="inside1"/>
-  <input type="radio" name="inside" id="inside2"/>
-  <input type="radio" name="inside" id="inside3"/>
-</form>
-<form id="before"></form>
-<input type="radio" form="before" name="before" id="before1"/>
-<input type="radio" form="before" name="before" id="before2"/>
-<input type="radio" form="before" name="before" id="before3"/>
-<input type="radio" form="after" name="after" id="after1"/>
-<input type="radio" form="after" name="after" id="after2"/>
-<input type="radio" form="after" name="after" id="after3"/>
-<form id="after"></form>
-<input type="radio" name="mix" id="mix1"/>
-<form id="mix"><input type="radio" name="mix" id="mix2"/></form>
-<input type="radio" name="mix" id="mix3"/>
-<input type="radio" name="doc" id="doc1"/>
-<input type="radio" name="doc" id="doc2"/>
-<input type="radio" name="doc" id="doc3"/>
-<script>
-async function pressRight() {
-  return new test_driver.Actions()
-      .keyDown("\uE014")
-      .keyUp("\uE014")
-      .send();
-}
-
-promise_test(async () => {
-  for (const groupName of ["inside", "before", "after", "mix", "doc"]) {
-    const firstInGroup = document.querySelector(`input[name="${groupName}"]`);
-    const newInput = document.createElement("input");
-    newInput.id = groupName + "New";
-    newInput.type = "radio";
-    if (groupName != "doc") {
-      newInput.setAttribute("form", groupName);
-    }
-    newInput.name = groupName;
-    firstInGroup.after(newInput);
-  }
-
-  for (const formId of ["inside", "before", "after", "mix"]) {
-    document.forms[formId].elements[0].focus();
-    for (const radio of document.forms[formId].elements) {
-      assert_equals(radio, document.activeElement, `Navigated to next radio button in form '${formId}'`);
-      await pressRight();
-    }
-  }
-
-  const radios = document.querySelectorAll("input[name='doc']");
-  radios[0].focus();
-  for (const radio of radios) {
-    assert_equals(radio, document.activeElement, `Navigated to next radio button on document`);
-    await pressRight();
-  }
-}, "Radio button keyboard navigation should proceed in tree-order.");
-</script>
-</body>
-</html>