diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index f1ab4515f0a7..c9ba3ee1ff20 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -688,10 +688,8 @@ CustomElementRegistry::Define(const nsAString& aName,
    * 2. If name is not a valid custom element name, then throw a "SyntaxError"
    *    DOMException and abort these steps.
    */
-  nsIDocument* doc = mWindow->GetExtantDoc();
-  uint32_t nameSpaceID = doc ? doc->GetDefaultNamespaceID() : kNameSpaceID_XHTML;
   RefPtr<nsAtom> nameAtom(NS_Atomize(aName));
-  if (!nsContentUtils::IsCustomElementName(nameAtom, nameSpaceID)) {
+  if (!nsContentUtils::IsCustomElementName(nameAtom)) {
     aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
     return;
   }
@@ -733,7 +731,7 @@ CustomElementRegistry::Define(const nsAString& aName,
   nsAutoString localName(aName);
   if (aOptions.mExtends.WasPassed()) {
     RefPtr<nsAtom> extendsAtom(NS_Atomize(aOptions.mExtends.Value()));
-    if (nsContentUtils::IsCustomElementName(extendsAtom, nameSpaceID)) {
+    if (nsContentUtils::IsCustomElementName(extendsAtom)) {
       aRv.Throw(NS_ERROR_DOM_NOT_SUPPORTED_ERR);
       return;
     }
@@ -973,9 +971,7 @@ CustomElementRegistry::WhenDefined(const nsAString& aName, ErrorResult& aRv)
   }
 
   RefPtr<nsAtom> nameAtom(NS_Atomize(aName));
-  nsIDocument* doc = mWindow->GetExtantDoc();
-  uint32_t nameSpaceID = doc ? doc->GetDefaultNamespaceID() : kNameSpaceID_XHTML;
-  if (!nsContentUtils::IsCustomElementName(nameAtom, nameSpaceID)) {
+  if (!nsContentUtils::IsCustomElementName(nameAtom)) {
     promise->MaybeReject(NS_ERROR_DOM_SYNTAX_ERR);
     return promise.forget();
   }
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 3ecff81743e2..485f6841bf67 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1196,7 +1196,7 @@ Element::AttachShadow(const ShadowRootInit& aInit, ErrorResult& aError)
    *    then throw a "NotSupportedError" DOMException.
    */
   nsAtom* nameAtom = NodeInfo()->NameAtom();
-  if (!(nsContentUtils::IsCustomElementName(nameAtom, NodeInfo()->NamespaceID()) ||
+  if (!(nsContentUtils::IsCustomElementName(nameAtom) ||
         nameAtom == nsGkAtoms::article ||
         nameAtom == nsGkAtoms::aside ||
         nameAtom == nsGkAtoms::blockquote ||
@@ -4304,7 +4304,7 @@ Element::SetCustomElementData(CustomElementData* aData)
   #if DEBUG
     nsAtom* name = NodeInfo()->NameAtom();
     nsAtom* type = aData->GetCustomElementType();
-    if (nsContentUtils::IsCustomElementName(name, NodeInfo()->NamespaceID())) {
+    if (nsContentUtils::IsCustomElementName(name)) {
       MOZ_ASSERT(type == name);
     } else {
       MOZ_ASSERT(type != name);
diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 090b0be55192..1415e5360c58 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -3194,13 +3194,8 @@ nsContentUtils::NewURIWithDocumentCharset(nsIURI** aResult,
 
 // static
 bool
-nsContentUtils::IsCustomElementName(nsAtom* aName, uint32_t aNameSpaceID)
+nsContentUtils::IsCustomElementName(nsAtom* aName)
 {
-  // Allow non-dashed names in XUL for XBL to Custom Element migrations.
-  if (aNameSpaceID == kNameSpaceID_XUL) {
-    return true;
-  }
-
   // A valid custom element name is a sequence of characters name which
   // must match the PotentialCustomElementName production:
   // PotentialCustomElementName ::= [a-z] (PCENChar)* '-' (PCENChar)*
@@ -9985,9 +9980,9 @@ nsContentUtils::NewXULOrHTMLElement(Element** aResult, mozilla::dom::NodeInfo* a
   if (nodeInfo->NamespaceEquals(kNameSpaceID_XHTML)) {
     tag = nsHTMLTags::CaseSensitiveAtomTagToId(name);
     isCustomElementName = (tag == eHTMLTag_userdefined &&
-                           nsContentUtils::IsCustomElementName(name, kNameSpaceID_XHTML));
+                           nsContentUtils::IsCustomElementName(name));
   } else {
-    isCustomElementName = nsContentUtils::IsCustomElementName(name, kNameSpaceID_XUL);
+    isCustomElementName = nsContentUtils::IsCustomElementName(name);
   }
 
   RefPtr<nsAtom> tagAtom = nodeInfo->NameAtom();
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index c0ba691dd3e1..227e2697ad06 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -710,7 +710,7 @@ public:
    * Returns true if |aName| is a valid name to be registered via
    * customElements.define.
    */
-  static bool IsCustomElementName(nsAtom* aName, uint32_t aNameSpaceID);
+  static bool IsCustomElementName(nsAtom* aName);
 
   static nsresult CheckQName(const nsAString& aQualifiedName,
                              bool aNamespaceAware = true,
diff --git a/dom/tests/mochitest/webcomponents/test_xul_custom_element.xul b/dom/tests/mochitest/webcomponents/test_xul_custom_element.xul
index 179779d1e555..5b33116f7f4c 100644
--- a/dom/tests/mochitest/webcomponents/test_xul_custom_element.xul
+++ b/dom/tests/mochitest/webcomponents/test_xul_custom_element.xul
@@ -26,9 +26,6 @@
 
     customElements.define("test-custom-element", TestCustomElement);
 
-    class TestWithoutDash extends XULElement { }
-    customElements.define("testwithoutdash", TestWithoutDash);
-
     function runTest() {
       const XUL_NS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";
 
@@ -56,9 +53,6 @@
          "Parser should have instantiated the custom element.");
       ok(element4 instanceof TestCustomElement, "Should be an instance of TestCustomElement");
 
-      let element5 = document.getElementById("element5");
-      ok(element5 instanceof TestWithoutDash, "Should be an instance of TestWithoutDash");
-
       SimpleTest.finish();
     }
   ]]>
@@ -68,7 +62,6 @@
     <p id="display"></p>
     <div id="content" style="display: none">
       <test-custom-element id="element4" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"/>
-      <testwithoutdash id="element5" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"/>
     </div>
     <pre id="test"></pre>
   </body>