diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 08e130744069..6a6726acd10d 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1200,26 +1200,15 @@ Element::GetShadowRootByMode() const
   return shadowRoot;
 }
 
-// https://dom.spec.whatwg.org/#dom-element-attachshadow
-already_AddRefed<ShadowRoot>
-Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
+bool
+Element::CanAttachShadowDOM() const
 {
   /**
-   * 1. If context object’s namespace is not the HTML namespace,
-   *    then throw a "NotSupportedError" DOMException.
-   */
-  if (!IsHTMLElement() &&
-      !(XRE_IsParentProcess() && IsXULElement() && nsContentUtils::AllowXULXBLForPrincipal(NodePrincipal()))) {
-    aError.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
-    return nullptr;
-  }
-
-  /**
-   * 2. If context object’s local name is not
-   *      a valid custom element name, "article", "aside", "blockquote",
-   *      "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6",
-   *      "header", "main" "nav", "p", "section", or "span",
-   *    then throw a "NotSupportedError" DOMException.
+   * If context object’s local name is not
+   *    a valid custom element name, "article", "aside", "blockquote",
+   *    "body", "div", "footer", "h1", "h2", "h3", "h4", "h5", "h6",
+   *    "header", "main" "nav", "p", "section", or "span",
+   *  return false.
    */
   nsAtom* nameAtom = NodeInfo()->NameAtom();
   if (!(nsContentUtils::IsCustomElementName(nameAtom, NodeInfo()->NamespaceID()) ||
@@ -1241,12 +1230,37 @@ Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
         nameAtom == nsGkAtoms::p ||
         nameAtom == nsGkAtoms::section ||
         nameAtom == nsGkAtoms::span)) {
+    return false;
+  }
+
+  return true;
+}
+
+// https://dom.spec.whatwg.org/#dom-element-attachshadow
+already_AddRefed<ShadowRoot>
+Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
+{
+  /**
+   * 1. If context object’s namespace is not the HTML namespace,
+   *    then throw a "NotSupportedError" DOMException.
+   */
+  if (!IsHTMLElement() &&
+      !(XRE_IsParentProcess() && IsXULElement() && nsContentUtils::AllowXULXBLForPrincipal(NodePrincipal()))) {
     aError.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
     return nullptr;
   }
 
   /**
-   * 3. If context object is a shadow host, then throw
+   * 2. If context object’s local name is not valid to attach shadow DOM to,
+   *    then throw a "NotSupportedError" DOMException.
+   */
+  if (!CanAttachShadowDOM()) {
+    aError.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
+    return nullptr;
+  }
+
+  /**
+   * 2. If context object is a shadow host, then throw
    *    an "InvalidStateError" DOMException.
    */
   if (GetShadowRoot() || GetXBLBinding()) {
diff --git a/dom/base/Element.h b/dom/base/Element.h
index 06461bab39dc..527800aa572a 100644
--- a/dom/base/Element.h
+++ b/dom/base/Element.h
@@ -1267,6 +1267,7 @@ public:
   // Shadow DOM v1
   already_AddRefed<ShadowRoot> AttachShadow(const ShadowRootInit& aInit,
                                             ErrorResult& aError);
+  bool CanAttachShadowDOM() const;
 
   already_AddRefed<ShadowRoot> AttachShadowWithoutNameChecks(ShadowRootMode aMode);
   void UnattachShadow();
diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index d72772c55a71..2f8db4817024 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -4932,20 +4932,6 @@ HTMLMediaElement::AssertReadyStateIsNothing()
 #endif
 }
 
-void
-HTMLMediaElement::AttachAndSetUAShadowRoot()
-{
-  if (GetShadowRoot()) {
-    MOZ_ASSERT(GetShadowRoot()->IsUAWidget());
-    return;
-  }
-
-  // Add a closed shadow root to host video controls
-  RefPtr<ShadowRoot> shadowRoot =
-    AttachShadowWithoutNameChecks(ShadowRootMode::Closed);
-  shadowRoot->SetIsUAWidget();
-}
-
 nsresult
 HTMLMediaElement::InitializeDecoderAsClone(ChannelMediaDecoder* aOriginal)
 {
diff --git a/dom/html/HTMLMediaElement.h b/dom/html/HTMLMediaElement.h
index 7ecebec658f7..d12a6416b83c 100644
--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -1903,9 +1903,6 @@ private:
   // For debugging bug 1407148.
   void AssertReadyStateIsNothing();
 
-  // Attach UA Shadow Root if it is not attached.
-  void AttachAndSetUAShadowRoot();
-
   // Contains the unique id of the sink device and the device info.
   // The initial value is ("", nullptr) and the default output device is used.
   // It can contain an invalid id and info if the device has been
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index b4de2b38872e..ec023505dac2 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -2892,6 +2892,21 @@ nsGenericHTMLElement::IsEventAttributeNameInternal(nsAtom *aName)
   return nsContentUtils::IsEventAttributeName(aName, EventNameType_HTML);
 }
 
+void
+nsGenericHTMLElement::AttachAndSetUAShadowRoot()
+{
+  MOZ_DIAGNOSTIC_ASSERT(!CanAttachShadowDOM(),
+                        "Cannot be used to attach UI shadow DOM");
+  if (GetShadowRoot()) {
+    MOZ_ASSERT(GetShadowRoot()->IsUAWidget());
+    return;
+  }
+
+  RefPtr<ShadowRoot> shadowRoot =
+    AttachShadowWithoutNameChecks(ShadowRootMode::Closed);
+  shadowRoot->SetIsUAWidget();
+}
+
 /**
  * Construct a URI from a string, as an element.src attribute
  * would be set to. Helper for the media elements.
diff --git a/dom/html/nsGenericHTMLElement.h b/dom/html/nsGenericHTMLElement.h
index 4bd3be4e7a9e..37d068c6daa0 100644
--- a/dom/html/nsGenericHTMLElement.h
+++ b/dom/html/nsGenericHTMLElement.h
@@ -256,6 +256,9 @@ public:
     return IsNodeInternal(aFirst, aArgs...);
   }
 
+  // Attach UA Shadow Root if it is not attached.
+  void AttachAndSetUAShadowRoot();
+
 protected:
   virtual ~nsGenericHTMLElement() {}