diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 674d7ef99cb3a..f8cc443b88bfb 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -1143,6 +1143,10 @@ static void DoUpgrade(Element* aElement, CustomElementDefinition* aDefinition,
     return;
   }
 
+  RefPtr<CustomElementData> data = aElement->GetCustomElementData();
+  MOZ_ASSERT(data, "CustomElementData should exist");
+  data->mState = CustomElementData::State::ePrecustomized;
+
   JS::Rooted<JS::Value> constructResult(RootingCx());
   // Rethrow the exception since it might actually throw the exception from the
   // upgrade steps back out to the caller of document.createElement.
@@ -1223,7 +1227,13 @@ void CustomElementRegistry::Upgrade(Element* aElement,
   DoUpgrade(aElement, aDefinition, MOZ_KnownLive(aDefinition->mConstructor),
             aRv);
   if (aRv.Failed()) {
-    MOZ_ASSERT(data->mState == CustomElementData::State::eFailed);
+    MOZ_ASSERT(data->mState == CustomElementData::State::eFailed ||
+               data->mState == CustomElementData::State::ePrecustomized);
+    // Spec doesn't set custom element state to failed here, but without this we
+    // would have inconsistent state on a custom elemet that is failed to
+    // upgrade, see https://github.com/whatwg/html/issues/6929, and
+    // https://github.com/web-platform-tests/wpt/pull/29911 for the test.
+    data->mState = CustomElementData::State::eFailed;
     aElement->SetCustomElementDefinition(nullptr);
     // Empty element's custom element reaction queue.
     data->mReactionQueue.Clear();
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index 16d3c3dfd3dd9..87d74ebcb68b7 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -97,7 +97,7 @@ struct CustomElementData {
   // CustomElementData is only created on the element which is a custom element
   // or an upgrade candidate, so the state of an element without
   // CustomElementData is "uncustomized".
-  enum class State { eUndefined, eFailed, eCustom };
+  enum class State { eUndefined, eFailed, eCustom, ePrecustomized };
 
   explicit CustomElementData(nsAtom* aType);
   CustomElementData(nsAtom* aType, State aState);
diff --git a/dom/html/nsGenericHTMLElement.cpp b/dom/html/nsGenericHTMLElement.cpp
index 37e864ea31221..58affa4e3c273 100644
--- a/dom/html/nsGenericHTMLElement.cpp
+++ b/dom/html/nsGenericHTMLElement.cpp
@@ -2882,7 +2882,7 @@ void nsGenericHTMLElement::SetInnerText(const nsAString& aValue) {
   mb.NodesAdded();
 }
 
-// https://html.spec.whatwg.org/commit-snapshots/b48bb2238269d90ea4f455a52cdf29505aff3df0/#dom-attachinternals
+// https://html.spec.whatwg.org/commit-snapshots/53bc3803433e1c817918b83e8a84f3db900031dd/#dom-attachinternals
 already_AddRefed<ElementInternals> nsGenericHTMLElement::AttachInternals(
     ErrorResult& aRv) {
   CustomElementData* ceData = GetCustomElementData();
@@ -2932,6 +2932,10 @@ already_AddRefed<ElementInternals> nsGenericHTMLElement::AttachInternals(
     return nullptr;
   }
 
+  // If this is not a custom element, i.e. ceData is nullptr, we are unable to
+  // find a definition and should return earlier above.
+  MOZ_ASSERT(ceData);
+
   // 5. If element's attached internals is true, then throw an
   //    "NotSupportedError" DOMException.
   if (ceData->HasAttachedInternals()) {
@@ -2941,10 +2945,19 @@ already_AddRefed<ElementInternals> nsGenericHTMLElement::AttachInternals(
     return nullptr;
   }
 
-  // 6. Set element's attached internals to true.
+  // 6. If element's custom element state is not "precustomized" or "custom",
+  //    then throw a "NotSupportedError" DOMException.
+  if (ceData->mState != CustomElementData::State::ePrecustomized &&
+      ceData->mState != CustomElementData::State::eCustom) {
+    aRv.ThrowNotSupportedError(
+        R"(Custom element state is not "precustomized" or "custom".)");
+    return nullptr;
+  }
+
+  // 7. Set element's attached internals to true.
   ceData->AttachedInternals();
 
-  // 7. Create a new ElementInternals instance targeting element, and return it.
+  // 8. Create a new ElementInternals instance targeting element, and return it.
   return MakeAndAddRef<ElementInternals>(this);
 }