diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 940c5d721ce2..76ebf29b85d8 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -636,6 +636,61 @@ int32_t CustomElementRegistry::InferNamespace(
   return kNameSpaceID_XHTML;
 }
 
+bool CustomElementRegistry::JSObjectToAtomArray(
+    JSContext* aCx, JS::Handle<JSObject*> aConstructor, const char16_t* aName,
+    nsTArray<RefPtr<nsAtom>>& aArray, ErrorResult& aRv) {
+  JS::RootedValue iterable(aCx, JS::UndefinedValue());
+  if (!JS_GetUCProperty(aCx, aConstructor, aName,
+                        std::char_traits<char16_t>::length(aName), &iterable)) {
+    aRv.NoteJSContextException(aCx);
+    return false;
+  }
+
+  if (!iterable.isUndefined()) {
+    if (!iterable.isObject()) {
+      aRv.ThrowTypeError<MSG_NOT_SEQUENCE>(nsDependentString(aName));
+      return false;
+    }
+
+    JS::ForOfIterator iter(aCx);
+    if (!iter.init(iterable, JS::ForOfIterator::AllowNonIterable)) {
+      aRv.NoteJSContextException(aCx);
+      return false;
+    }
+
+    if (!iter.valueIsIterable()) {
+      aRv.ThrowTypeError<MSG_NOT_SEQUENCE>(nsDependentString(aName));
+      return false;
+    }
+
+    JS::Rooted<JS::Value> attribute(aCx);
+    while (true) {
+      bool done;
+      if (!iter.next(&attribute, &done)) {
+        aRv.NoteJSContextException(aCx);
+        return false;
+      }
+      if (done) {
+        break;
+      }
+
+      nsAutoString attrStr;
+      if (!ConvertJSValueToString(aCx, attribute, eStringify, eStringify,
+                                  attrStr)) {
+        aRv.NoteJSContextException(aCx);
+        return false;
+      }
+
+      if (!aArray.AppendElement(NS_Atomize(attrStr))) {
+        aRv.Throw(NS_ERROR_OUT_OF_MEMORY);
+        return false;
+      }
+    }
+  }
+
+  return true;
+}
+
 // https://html.spec.whatwg.org/multipage/scripting.html#element-definition
 void CustomElementRegistry::Define(
     JSContext* aCx, const nsAString& aName,
@@ -782,6 +837,8 @@ void CustomElementRegistry::Define(
 
   auto callbacksHolder = MakeUnique<LifecycleCallbacks>();
   nsTArray<RefPtr<nsAtom>> observedAttributes;
+  AutoTArray<RefPtr<nsAtom>, 2> disabledFeatures;
+  bool disableInternals = false;
   {  // Set mIsCustomDefinitionRunning.
     /**
      * 9. Set this CustomElementRegistry's element definition is running flag.
@@ -841,58 +898,31 @@ void CustomElementRegistry::Define(
      *          any exceptions from the conversion.
      */
     if (callbacksHolder->mAttributeChangedCallback.WasPassed()) {
-      JS::Rooted<JS::Value> observedAttributesIterable(aCx);
+      if (!JSObjectToAtomArray(aCx, constructor, u"observedAttributes",
+                               observedAttributes, aRv)) {
+        return;
+      }
+    }
 
-      if (!JS_GetProperty(aCx, constructor, "observedAttributes",
-                          &observedAttributesIterable)) {
-        aRv.NoteJSContextException(aCx);
+    /**
+     * 14.6. Let disabledFeatures be an empty sequence<DOMString>.
+     * 14.7. Let disabledFeaturesIterable be Get(constructor,
+     *       "disabledFeatures"). Rethrow any exceptions.
+     * 14.8. If disabledFeaturesIterable is not undefined, then set
+     *       disabledFeatures to the result of converting
+     *       disabledFeaturesIterable to a sequence<DOMString>.
+     *       Rethrow any exceptions from the conversion.
+     */
+    if (StaticPrefs::dom_webcomponents_elementInternals_enabled()) {
+      if (!JSObjectToAtomArray(aCx, constructor, u"disabledFeatures",
+                               disabledFeatures, aRv)) {
         return;
       }
 
-      if (!observedAttributesIterable.isUndefined()) {
-        if (!observedAttributesIterable.isObject()) {
-          aRv.ThrowTypeError<MSG_NOT_SEQUENCE>(
-              NS_LITERAL_STRING("observedAttributes"));
-          return;
-        }
-
-        JS::ForOfIterator iter(aCx);
-        if (!iter.init(observedAttributesIterable,
-                       JS::ForOfIterator::AllowNonIterable)) {
-          aRv.NoteJSContextException(aCx);
-          return;
-        }
-
-        if (!iter.valueIsIterable()) {
-          aRv.ThrowTypeError<MSG_NOT_SEQUENCE>(
-              NS_LITERAL_STRING("observedAttributes"));
-          return;
-        }
-
-        JS::Rooted<JS::Value> attribute(aCx);
-        while (true) {
-          bool done;
-          if (!iter.next(&attribute, &done)) {
-            aRv.NoteJSContextException(aCx);
-            return;
-          }
-          if (done) {
-            break;
-          }
-
-          nsAutoString attrStr;
-          if (!ConvertJSValueToString(aCx, attribute, eStringify, eStringify,
-                                      attrStr)) {
-            aRv.NoteJSContextException(aCx);
-            return;
-          }
-
-          if (!observedAttributes.AppendElement(NS_Atomize(attrStr))) {
-            aRv.Throw(NS_ERROR_OUT_OF_MEMORY);
-            return;
-          }
-        }
-      }
+      // 14.9. Set disableInternals to true if disabledFeaturesSequence contains
+      //       "internals".
+      disableInternals = disabledFeatures.Contains(
+          static_cast<nsStaticAtom*>(nsGkAtoms::internals));
     }
   }  // Unset mIsCustomDefinitionRunning
 
@@ -915,7 +945,8 @@ void CustomElementRegistry::Define(
 
   RefPtr<CustomElementDefinition> definition = new CustomElementDefinition(
       nameAtom, localNameAtom, nameSpaceID, &aFunctionConstructor,
-      std::move(observedAttributes), std::move(callbacksHolder));
+      std::move(observedAttributes), std::move(callbacksHolder),
+      disableInternals);
 
   CustomElementDefinition* def = definition.get();
   mCustomDefinitions.Put(nameAtom, definition.forget());
@@ -1405,13 +1436,14 @@ CustomElementDefinition::CustomElementDefinition(
     nsAtom* aType, nsAtom* aLocalName, int32_t aNamespaceID,
     CustomElementConstructor* aConstructor,
     nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
-    UniquePtr<LifecycleCallbacks>&& aCallbacks)
+    UniquePtr<LifecycleCallbacks>&& aCallbacks, bool aDisableInternals)
     : mType(aType),
       mLocalName(aLocalName),
       mNamespaceID(aNamespaceID),
       mConstructor(aConstructor),
       mObservedAttributes(std::move(aObservedAttributes)),
-      mCallbacks(std::move(aCallbacks)) {}
+      mCallbacks(std::move(aCallbacks)),
+      mDisableInternals(aDisableInternals) {}
 
 }  // namespace dom
 }  // namespace mozilla
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index aaa4557c3bff..dc54a361d29c 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -137,7 +137,8 @@ struct CustomElementDefinition {
                           int32_t aNamespaceID,
                           CustomElementConstructor* aConstructor,
                           nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
-                          UniquePtr<LifecycleCallbacks>&& aCallbacks);
+                          UniquePtr<LifecycleCallbacks>&& aCallbacks,
+                          bool aDisableInternals);
 
   // The type (name) for this custom element, for <button is="x-foo"> or <x-foo>
   // this would be x-foo.
@@ -158,6 +159,9 @@ struct CustomElementDefinition {
   // The lifecycle callbacks to call for this custom element.
   UniquePtr<LifecycleCallbacks> mCallbacks;
 
+  // Determine whether to allow to attachInternals() for this custom element.
+  bool mDisableInternals = false;
+
   // A construction stack. Use nullptr to represent an "already constructed
   // marker".
   nsTArray<RefPtr<Element>> mConstructionStack;
@@ -458,6 +462,10 @@ class CustomElementRegistry final : public nsISupports, public nsWrapperCache {
  private:
   ~CustomElementRegistry();
 
+  bool JSObjectToAtomArray(JSContext* aCx, JS::Handle<JSObject*> aConstructor,
+                           const char16_t* aName,
+                           nsTArray<RefPtr<nsAtom>>& aArray, ErrorResult& aRv);
+
   static UniquePtr<CustomElementCallback> CreateCustomElementCallback(
       Document::ElementCallbackType aType, Element* aCustomElement,
       LifecycleCallbackArgs* aArgs,
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 4ce2a6a9b868..c265fae3d095 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -2620,6 +2620,12 @@
   value: false
   mirror: always
 
+# Is support for elementInternals enabled?
+- name: dom.webcomponents.elementInternals.enabled
+  type: bool
+  value: @IS_NIGHTLY_BUILD@
+  mirror: always
+
 # Is support for the Web GPU API enabled?
 - name: dom.webgpu.enable
   type: RelaxedAtomicBool
diff --git a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
index 8dd5b444b4f8..3de73c28ab26 100644
--- a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
+++ b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
@@ -17,18 +17,11 @@
   [customElements.define must rethrow an exception thrown while getting additional formAssociated callbacks on the constructor prototype]
     expected: FAIL
 
-  [customElements.define must rethrow an exception thrown while retrieving Symbol.iterator on disabledFeatures]
-    expected: FAIL
-
   [customElements.define must get four additional callbacks on the prototype if formAssociated is converted to true]
     expected: FAIL
 
-  [customElements.define must rethrow an exception thrown while getting disabledFeatures on the constructor prototype]
-    expected: FAIL
-
-  [customElements.define must rethrow an exception thrown while iterating over disabledFeatures to sequence<DOMString>]
-    expected: FAIL
-
-  [customElements.define must rethrow an exception thrown while converting the value of disabledFeatures to sequence<DOMString>]
+  [customElements.define must not throw when defining another custom element in a different global object during Get(constructor, "prototype")]
     expected: FAIL
 
+  [customElements.define must get "prototype" property of the constructor]
+    expected: FAIL
\ No newline at end of file
diff --git a/testing/web-platform/meta/custom-elements/__dir__.ini b/testing/web-platform/meta/custom-elements/__dir__.ini
index e69de29bb2d1..ee45da1ec0c0 100644
--- a/testing/web-platform/meta/custom-elements/__dir__.ini
+++ b/testing/web-platform/meta/custom-elements/__dir__.ini
@@ -0,0 +1 @@
+prefs: [dom.webcomponents.elementInternals.enabled:true]
\ No newline at end of file
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index 4998d0fb2266..ebd19e7d8f48 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -537,6 +537,7 @@ STATIC_ATOMS = [
     Atom("insertion", "insertion"),
     Atom("integer", "integer"),
     Atom("integrity", "integrity"),
+    Atom("internals", "internals"),
     Atom("intersection", "intersection"),
     Atom("intersectionobserverlist", "intersectionobserverlist"),
     Atom("is", "is"),