diff --git a/dom/base/CustomElementRegistry.cpp b/dom/base/CustomElementRegistry.cpp
index 3ceb8e0c73d3..0fd8c2e14fe6 100644
--- a/dom/base/CustomElementRegistry.cpp
+++ b/dom/base/CustomElementRegistry.cpp
@@ -11,6 +11,7 @@
 #include "mozilla/dom/CustomElementRegistryBinding.h"
 #include "mozilla/dom/ElementBinding.h"
 #include "mozilla/dom/HTMLElementBinding.h"
+#include "mozilla/dom/PrimitiveConversions.h"
 #include "mozilla/dom/ShadowIncludingTreeIterator.h"
 #include "mozilla/dom/XULElementBinding.h"
 #include "mozilla/dom/Promise.h"
@@ -847,6 +848,7 @@ void CustomElementRegistry::Define(
   auto callbacksHolder = MakeUnique<LifecycleCallbacks>();
   nsTArray<RefPtr<nsAtom>> observedAttributes;
   AutoTArray<RefPtr<nsAtom>, 2> disabledFeatures;
+  bool formAssociated = false;
   bool disableInternals = false;
   bool disableShadow = false;
   {  // Set mIsCustomDefinitionRunning.
@@ -921,7 +923,7 @@ void CustomElementRegistry::Define(
      *       disabledFeaturesIterable to a sequence<DOMString>.
      *       Rethrow any exceptions from the conversion.
      */
-    if (StaticPrefs::dom_webcomponents_elementInternals_enabled()) {
+    if (StaticPrefs::dom_webcomponents_formAssociatedCustomElement_enabled()) {
       if (!JSObjectToAtomArray(aCx, constructor, u"disabledFeatures"_ns,
                                disabledFeatures, aRv)) {
         return;
@@ -936,6 +938,24 @@ void CustomElementRegistry::Define(
       //        "shadow".
       disableShadow = disabledFeatures.Contains(
           static_cast<nsStaticAtom*>(nsGkAtoms::shadow));
+
+      // 14.11. Let formAssociatedValue be Get(constructor, "formAssociated").
+      //        Rethrow any exceptions.
+      JS::Rooted<JS::Value> formAssociatedValue(aCx);
+      if (!JS_GetProperty(aCx, constructor, "formAssociated",
+                          &formAssociatedValue)) {
+        aRv.NoteJSContextException(aCx);
+        return;
+      }
+
+      // 14.12. Set formAssociated to the result of converting
+      //        formAssociatedValue to a boolean. Rethrow any exceptions from
+      //        the conversion.
+      if (!ValueToPrimitive<bool, eDefault>(
+              aCx, formAssociatedValue, "formAssociated", &formAssociated)) {
+        aRv.NoteJSContextException(aCx);
+        return;
+      }
     }
   }  // Unset mIsCustomDefinitionRunning
 
@@ -958,7 +978,7 @@ void CustomElementRegistry::Define(
 
   RefPtr<CustomElementDefinition> definition = new CustomElementDefinition(
       nameAtom, localNameAtom, nameSpaceID, &aFunctionConstructor,
-      std::move(observedAttributes), std::move(callbacksHolder),
+      std::move(observedAttributes), std::move(callbacksHolder), formAssociated,
       disableInternals, disableShadow);
 
   CustomElementDefinition* def = definition.get();
@@ -1470,14 +1490,15 @@ CustomElementDefinition::CustomElementDefinition(
     nsAtom* aType, nsAtom* aLocalName, int32_t aNamespaceID,
     CustomElementConstructor* aConstructor,
     nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
-    UniquePtr<LifecycleCallbacks>&& aCallbacks, bool aDisableInternals,
-    bool aDisableShadow)
+    UniquePtr<LifecycleCallbacks>&& aCallbacks, bool aFormAssociated,
+    bool aDisableInternals, bool aDisableShadow)
     : mType(aType),
       mLocalName(aLocalName),
       mNamespaceID(aNamespaceID),
       mConstructor(aConstructor),
       mObservedAttributes(std::move(aObservedAttributes)),
       mCallbacks(std::move(aCallbacks)),
+      mFormAssociated(aFormAssociated),
       mDisableInternals(aDisableInternals),
       mDisableShadow(aDisableShadow) {}
 
diff --git a/dom/base/CustomElementRegistry.h b/dom/base/CustomElementRegistry.h
index fac6485c54d1..a3b9726bfc93 100644
--- a/dom/base/CustomElementRegistry.h
+++ b/dom/base/CustomElementRegistry.h
@@ -141,7 +141,8 @@ struct CustomElementDefinition {
                           CustomElementConstructor* aConstructor,
                           nsTArray<RefPtr<nsAtom>>&& aObservedAttributes,
                           UniquePtr<LifecycleCallbacks>&& aCallbacks,
-                          bool aDisableInternals, bool aDisableShadow);
+                          bool aFormAssociated, bool aDisableInternals,
+                          bool aDisableShadow);
 
   // The type (name) for this custom element, for <button is="x-foo"> or <x-foo>
   // this would be x-foo.
@@ -162,6 +163,10 @@ struct CustomElementDefinition {
   // The lifecycle callbacks to call for this custom element.
   UniquePtr<LifecycleCallbacks> mCallbacks;
 
+  // If this is true, user agent treats elements associated to this custom
+  // element definition as form-associated custom elements.
+  bool mFormAssociated = false;
+
   // Determine whether to allow to attachInternals() for this custom element.
   bool mDisableInternals = false;
 
diff --git a/dom/base/Element.cpp b/dom/base/Element.cpp
index 5b707b4a7aec..36174035f01e 100644
--- a/dom/base/Element.cpp
+++ b/dom/base/Element.cpp
@@ -1029,7 +1029,8 @@ bool Element::CanAttachShadowDOM() const {
   // It will always have CustomElementData when the element is a valid custom
   // element or has is value.
   CustomElementData* ceData = GetCustomElementData();
-  if (StaticPrefs::dom_webcomponents_elementInternals_enabled() && ceData) {
+  if (StaticPrefs::dom_webcomponents_formAssociatedCustomElement_enabled() &&
+      ceData) {
     CustomElementDefinition* definition = ceData->GetCustomElementDefinition();
     // If the definition is null, the element possible hasn't yet upgraded.
     // Fallback to use LookupCustomElementDefinition to find its definition.
diff --git a/dom/webidl/ElementInternals.webidl b/dom/webidl/ElementInternals.webidl
index 5cec7db7428b..c63c46d186d1 100644
--- a/dom/webidl/ElementInternals.webidl
+++ b/dom/webidl/ElementInternals.webidl
@@ -7,7 +7,7 @@
  * https://html.spec.whatwg.org/#elementinternals
  */
 
-[Pref="dom.webcomponents.elementInternals.enabled", Exposed=Window]
+[Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Exposed=Window]
 interface ElementInternals {
 };
 
diff --git a/dom/webidl/HTMLElement.webidl b/dom/webidl/HTMLElement.webidl
index b6afe0d9a9c0..b8dfdc97422b 100644
--- a/dom/webidl/HTMLElement.webidl
+++ b/dom/webidl/HTMLElement.webidl
@@ -68,7 +68,7 @@ interface HTMLElement : Element {
   //readonly attribute boolean? commandChecked;
 
   // https://html.spec.whatwg.org/multipage/custom-elements.html#dom-attachinternals
-  [Pref="dom.webcomponents.elementInternals.enabled", Throws]
+  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
   ElementInternals attachInternals();
 };
 
diff --git a/modules/libpref/init/StaticPrefList.yaml b/modules/libpref/init/StaticPrefList.yaml
index 05c4ffec4b5d..ed3f3a81172f 100644
--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -3232,8 +3232,8 @@
   value: false
   mirror: always
 
-# Is support for elementInternals enabled?
-- name: dom.webcomponents.elementInternals.enabled
+# Is support for form-associated custom element enabled?
+- name: dom.webcomponents.formAssociatedCustomElement.enabled
   type: bool
   value: @IS_NIGHTLY_BUILD@
   mirror: always
diff --git a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
index 38c0c04ad388..8649faec3cca 100644
--- a/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
+++ b/testing/web-platform/meta/custom-elements/CustomElementRegistry.html.ini
@@ -11,18 +11,9 @@
   [customElements.define must get "observedAttributes" property on the constructor prototype when "attributeChangedCallback" is present]
     expected: FAIL
 
-  [customElements.define must rethrow an exception thrown while getting formAssociated on the constructor prototype]
-    expected: FAIL
-
   [customElements.define must rethrow an exception thrown while getting additional formAssociated callbacks on the constructor prototype]
     expected: FAIL
 
   [customElements.define must get four additional callbacks on the prototype if formAssociated is converted to true]
     expected: FAIL
 
-  [customElements.define must get "prototype", "disabledFeatures", and "formAssociated" property of the constructor]
-    expected: FAIL
-
-  [customElements.define must not throw when defining another custom element in a different global object during Get(constructor, "prototype")]
-    expected: FAIL
-
diff --git a/testing/web-platform/meta/custom-elements/__dir__.ini b/testing/web-platform/meta/custom-elements/__dir__.ini
index a8763f34a6c6..52b5abbc1a3c 100644
--- a/testing/web-platform/meta/custom-elements/__dir__.ini
+++ b/testing/web-platform/meta/custom-elements/__dir__.ini
@@ -1,2 +1,2 @@
-prefs: [dom.webcomponents.elementInternals.enabled:true]
+prefs: [dom.webcomponents.formAssociatedCustomElement.enabled:true]
 leak-threshold: [default:51200]
diff --git a/testing/web-platform/meta/html/dom/idlharness.https.html.ini b/testing/web-platform/meta/html/dom/idlharness.https.html.ini
index eecc5d7e8dca..acb69db68219 100644
--- a/testing/web-platform/meta/html/dom/idlharness.https.html.ini
+++ b/testing/web-platform/meta/html/dom/idlharness.https.html.ini
@@ -1,4 +1,4 @@
-prefs: [dom.security.featurePolicy.enabled:true, dom.security.featurePolicy.experimental.enabled:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true, dom.webcomponents.elementInternals.enabled:true, dom.forms.requestsubmit.enabled:true, dom.forms.inputmode:true, dom.forms.enterkeyhint:true]
+prefs: [dom.security.featurePolicy.enabled:true, dom.security.featurePolicy.experimental.enabled:true, dom.security.featurePolicy.header.enabled:true, dom.security.featurePolicy.webidl.enabled:true, dom.webcomponents.formAssociatedCustomElement.enabled:true, dom.forms.requestsubmit.enabled:true, dom.forms.inputmode:true, dom.forms.enterkeyhint:true]
 [idlharness.https.html?exclude=(Document|Window|HTML.*)]
   [OffscreenCanvasRenderingContext2D interface: operation lineTo(unrestricted double, unrestricted double)]
     expected: FAIL
diff --git a/testing/web-platform/meta/shadow-dom/__dir__.ini b/testing/web-platform/meta/shadow-dom/__dir__.ini
index d7a893099a5e..1b4348cbc207 100644
--- a/testing/web-platform/meta/shadow-dom/__dir__.ini
+++ b/testing/web-platform/meta/shadow-dom/__dir__.ini
@@ -1 +1 @@
-prefs: [dom.webcomponents.elementInternals.enabled:true]
+prefs: [dom.webcomponents.formAssociatedCustomElement.enabled:true]