diff --git a/dom/html/ElementInternals.cpp b/dom/html/ElementInternals.cpp
index 3c0dfe0fe976..dd0221825034 100644
--- a/dom/html/ElementInternals.cpp
+++ b/dom/html/ElementInternals.cpp
@@ -24,6 +24,7 @@ NS_IMPL_CYCLE_COLLECTING_RELEASE(ElementInternals)
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(ElementInternals)
   NS_WRAPPERCACHE_INTERFACE_MAP_ENTRY
   NS_INTERFACE_MAP_ENTRY(nsIFormControl)
+  NS_INTERFACE_MAP_ENTRY(nsIConstraintValidation)
 NS_INTERFACE_MAP_END
 
 ElementInternals::ElementInternals(HTMLElement* aTarget)
@@ -123,6 +124,16 @@ HTMLFormElement* ElementInternals::GetForm(ErrorResult& aRv) const {
   return GetForm();
 }
 
+// https://html.spec.whatwg.org/#dom-elementinternals-willvalidate
+bool ElementInternals::GetWillValidate(ErrorResult& aRv) const {
+  if (!mTarget || !mTarget->IsFormAssociatedElement()) {
+    aRv.ThrowNotSupportedError(
+        "Target element is not a form-associated custom element");
+    return false;
+  }
+  return WillValidate();
+}
+
 // https://html.spec.whatwg.org/#dom-elementinternals-labels
 already_AddRefed<nsINodeList> ElementInternals::GetLabels(
     ErrorResult& aRv) const {
@@ -188,6 +199,16 @@ void ElementInternals::UpdateFormOwner() {
   }
 }
 
+void ElementInternals::UpdateBarredFromConstraintValidation() {
+  if (mTarget) {
+    MOZ_ASSERT(mTarget->IsFormAssociatedElement());
+    SetBarredFromConstraintValidation(
+        mTarget->HasAttr(nsGkAtoms::readonly) ||
+        mTarget->HasFlag(ELEMENT_IS_DATALIST_OR_HAS_DATALIST_ANCESTOR) ||
+        mTarget->IsDisabled());
+  }
+}
+
 void ElementInternals::Unlink() {
   if (mForm) {
     // Don't notify, since we're being destroyed in any case.
diff --git a/dom/html/ElementInternals.h b/dom/html/ElementInternals.h
index a081db7dc4d0..a283e46af381 100644
--- a/dom/html/ElementInternals.h
+++ b/dom/html/ElementInternals.h
@@ -10,6 +10,7 @@
 #include "js/TypeDecls.h"
 #include "mozilla/dom/ElementInternalsBinding.h"
 #include "nsCycleCollectionParticipant.h"
+#include "nsIConstraintValidation.h"
 #include "nsIFormControl.h"
 #include "nsWrapperCache.h"
 
@@ -26,10 +27,13 @@ class HTMLFieldSetElement;
 class HTMLFormElement;
 class ShadowRoot;
 
-class ElementInternals final : public nsIFormControl, public nsWrapperCache {
+class ElementInternals final : public nsIFormControl,
+                               public nsIConstraintValidation,
+                               public nsWrapperCache {
  public:
   NS_DECL_CYCLE_COLLECTING_ISUPPORTS
-  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS(ElementInternals)
+  NS_DECL_CYCLE_COLLECTION_SCRIPT_HOLDER_CLASS_AMBIGUOUS(ElementInternals,
+                                                         nsIFormControl)
 
   explicit ElementInternals(HTMLElement* aTarget);
 
@@ -44,6 +48,7 @@ class ElementInternals final : public nsIFormControl, public nsWrapperCache {
                     const Optional<Nullable<FileOrUSVStringOrFormData>>& aState,
                     ErrorResult& aRv);
   mozilla::dom::HTMLFormElement* GetForm(ErrorResult& aRv) const;
+  bool GetWillValidate(ErrorResult& aRv) const;
   already_AddRefed<nsINodeList> GetLabels(ErrorResult& aRv) const;
 
   // nsIFormControl
@@ -62,6 +67,7 @@ class ElementInternals final : public nsIFormControl, public nsWrapperCache {
   }
 
   void UpdateFormOwner();
+  void UpdateBarredFromConstraintValidation();
 
   void Unlink();
 
diff --git a/dom/html/HTMLElement.cpp b/dom/html/HTMLElement.cpp
index f63e952c60a7..a35a3b346d76 100644
--- a/dom/html/HTMLElement.cpp
+++ b/dom/html/HTMLElement.cpp
@@ -25,6 +25,7 @@ NS_IMPL_CYCLE_COLLECTION_INHERITED(HTMLElement, nsGenericHTMLFormElement)
 
 NS_INTERFACE_MAP_BEGIN_CYCLE_COLLECTION(HTMLElement)
   NS_INTERFACE_MAP_ENTRY_TEAROFF(nsIFormControl, GetElementInternals())
+  NS_INTERFACE_MAP_ENTRY_TEAROFF(nsIConstraintValidation, GetElementInternals())
 NS_INTERFACE_MAP_END_INHERITING(nsGenericHTMLFormElement)
 
 NS_IMPL_ADDREF_INHERITED(HTMLElement, nsGenericHTMLFormElement)
@@ -37,6 +38,20 @@ JSObject* HTMLElement::WrapNode(JSContext* aCx,
   return dom::HTMLElement_Binding::Wrap(aCx, this, aGivenProto);
 }
 
+nsresult HTMLElement::BindToTree(BindContext& aContext, nsINode& aParent) {
+  nsresult rv = nsGenericHTMLFormElement::BindToTree(aContext, aParent);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  UpdateBarredFromConstraintValidation();
+  return rv;
+}
+
+void HTMLElement::UnbindFromTree(bool aNullParent) {
+  nsGenericHTMLFormElement::UnbindFromTree(aNullParent);
+
+  UpdateBarredFromConstraintValidation();
+}
+
 void HTMLElement::SetCustomElementDefinition(
     CustomElementDefinition* aDefinition) {
   nsGenericHTMLFormElement::SetCustomElementDefinition(aDefinition);
@@ -161,6 +176,7 @@ void HTMLElement::UpdateFormOwner() {
   }
   UpdateFieldSet(true);
   UpdateDisabledState(true);
+  UpdateBarredFromConstraintValidation();
 }
 
 nsresult HTMLElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
@@ -168,8 +184,14 @@ nsresult HTMLElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
                                    const nsAttrValue* aOldValue,
                                    nsIPrincipal* aMaybeScriptedPrincipal,
                                    bool aNotify) {
-  if (aNameSpaceID == kNameSpaceID_None && aName == nsGkAtoms::disabled) {
-    UpdateDisabledState(aNotify);
+  if (aNameSpaceID == kNameSpaceID_None &&
+      (aName == nsGkAtoms::disabled || aName == nsGkAtoms::readonly)) {
+    if (aName == nsGkAtoms::disabled) {
+      // This *has* to be called *before* validity state check because
+      // UpdateBarredFromConstraintValidation depend on our disabled state.
+      UpdateDisabledState(aNotify);
+    }
+    UpdateBarredFromConstraintValidation();
   }
 
   return nsGenericHTMLFormElement::AfterSetAttr(
@@ -239,6 +261,14 @@ bool HTMLElement::IsFormAssociatedElement() const {
   return isFormAssociatedCustomElement;
 }
 
+void HTMLElement::FieldSetDisabledChanged(bool aNotify) {
+  // This *has* to be called *before* UpdateBarredFromConstraintValidation
+  // because this function depend on our disabled state.
+  nsGenericHTMLFormElement::FieldSetDisabledChanged(aNotify);
+
+  UpdateBarredFromConstraintValidation();
+}
+
 ElementInternals* HTMLElement::GetElementInternals() const {
   CustomElementData* data = GetCustomElementData();
   if (!data || !data->IsFormAssociated()) {
@@ -251,6 +281,15 @@ ElementInternals* HTMLElement::GetElementInternals() const {
   return data->GetElementInternals();
 }
 
+void HTMLElement::UpdateBarredFromConstraintValidation() {
+  CustomElementData* data = GetCustomElementData();
+  if (data && data->IsFormAssociated()) {
+    ElementInternals* internals = data->GetElementInternals();
+    MOZ_ASSERT(internals);
+    internals->UpdateBarredFromConstraintValidation();
+  }
+}
+
 }  // namespace mozilla::dom
 
 // Here, we expand 'NS_IMPL_NS_NEW_HTML_ELEMENT()' by hand.
diff --git a/dom/html/HTMLElement.h b/dom/html/HTMLElement.h
index cb7c008156b7..2e23d5cc8c18 100644
--- a/dom/html/HTMLElement.h
+++ b/dom/html/HTMLElement.h
@@ -23,6 +23,10 @@ class HTMLElement final : public nsGenericHTMLFormElement {
   // nsINode
   nsresult Clone(dom::NodeInfo*, nsINode** aResult) const override;
 
+  // nsIContent
+  nsresult BindToTree(BindContext&, nsINode& aParent) override;
+  void UnbindFromTree(bool aNullParent = true) override;
+
   // Element
   void SetCustomElementDefinition(
       CustomElementDefinition* aDefinition) override;
@@ -36,6 +40,7 @@ class HTMLElement final : public nsGenericHTMLFormElement {
   // nsGenericHTMLFormElement
   bool IsFormAssociatedElement() const override;
   void AfterClearForm(bool aUnbindOrDelete) override;
+  void FieldSetDisabledChanged(bool aNotify) override;
 
   void UpdateFormOwner();
 
@@ -61,6 +66,8 @@ class HTMLElement final : public nsGenericHTMLFormElement {
   void UpdateDisabledState(bool aNotify) override;
   void UpdateFormOwner(bool aBindToTree, Element* aFormIdElement) override;
 
+  void UpdateBarredFromConstraintValidation();
+
   ElementInternals* GetElementInternals() const;
 };
 
diff --git a/dom/webidl/ElementInternals.webidl b/dom/webidl/ElementInternals.webidl
index 40eeb74cd8b4..44c65e78bb96 100644
--- a/dom/webidl/ElementInternals.webidl
+++ b/dom/webidl/ElementInternals.webidl
@@ -20,6 +20,9 @@ interface ElementInternals {
   [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
   readonly attribute HTMLFormElement? form;
 
+  [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
+  readonly attribute boolean willValidate;
+
   [Pref="dom.webcomponents.formAssociatedCustomElement.enabled", Throws]
   readonly attribute NodeList labels;
 };
diff --git a/testing/web-platform/meta/custom-elements/form-associated/ElementInternals-validation.html.ini b/testing/web-platform/meta/custom-elements/form-associated/ElementInternals-validation.html.ini
index d7bdad331752..d17749860e47 100644
--- a/testing/web-platform/meta/custom-elements/form-associated/ElementInternals-validation.html.ini
+++ b/testing/web-platform/meta/custom-elements/form-associated/ElementInternals-validation.html.ini
@@ -11,9 +11,6 @@
   [Custom control affects validation at the owner form]
     expected: FAIL
 
-  [willValidate]
-    expected: FAIL
-
   ["anchor" argument of setValidity()]
     expected: FAIL
 
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 39fc70a8302e..3214280a351c 100644
--- a/testing/web-platform/meta/html/dom/idlharness.https.html.ini
+++ b/testing/web-platform/meta/html/dom/idlharness.https.html.ini
@@ -372,7 +372,8 @@ prefs: [dom.security.featurePolicy.experimental.enabled:true, dom.security.featu
     expected: FAIL
 
   [ElementInternals interface: attribute willValidate]
-    expected: FAIL
+    expected:
+      if release_or_beta: FAIL
 
   [OffscreenCanvasRenderingContext2D interface: operation moveTo(unrestricted double, unrestricted double)]
     expected: FAIL
diff --git a/testing/web-platform/tests/custom-elements/form-associated/ElementInternals-validation.html b/testing/web-platform/tests/custom-elements/form-associated/ElementInternals-validation.html
index f67e96465bd6..6b2ef8739795 100644
--- a/testing/web-platform/tests/custom-elements/form-associated/ElementInternals-validation.html
+++ b/testing/web-platform/tests/custom-elements/form-associated/ElementInternals-validation.html
@@ -16,6 +16,15 @@ class MyControl extends HTMLElement {
 }
 customElements.define('my-control', MyControl);
 
+class NotFormAssociatedElement extends HTMLElement {
+  constructor() {
+    super();
+    this.internals_ = this.attachInternals();
+  }
+  get i() { return this.internals_; }
+}
+customElements.define('not-form-associated-element', NotFormAssociatedElement);
+
 test(() => {
   const control = new MyControl();
   assert_true(control.i.willValidate, 'default value is true');
@@ -23,18 +32,64 @@ test(() => {
   const datalist = document.createElement('datalist');
   datalist.appendChild(control);
   assert_false(control.i.willValidate, 'false in DATALIST');
+  datalist.removeChild(control);
+  assert_true(control.i.willValidate, 'remove from DATALIST');
 
   const fieldset = document.createElement('fieldset');
-  fieldset.appendChild(control);
-  assert_true(control.i.willValidate, 'In enabled FIELDSET');
   fieldset.disabled = true;
-  assert_false(control.i.willValidate, 'In disabled FIELDSET');
+  fieldset.appendChild(control);
+  assert_false(control.i.willValidate, 'append to disabled FIELDSET');
+  fieldset.disabled = false;
+  assert_true(control.i.willValidate, 'FIELDSET becomes enabled');
+  fieldset.disabled = true;
+  assert_false(control.i.willValidate, 'FIELDSET becomes disabled');
   fieldset.removeChild(control);
+  assert_true(control.i.willValidate, 'remove from disabled FIELDSET');
 
   control.setAttribute('disabled', '');
   assert_false(control.i.willValidate, 'with disabled attribute');
+  control.removeAttribute('disabled');
+  assert_true(control.i.willValidate, 'without disabled attribute');
+
+  control.setAttribute('readonly', '');
+  assert_false(control.i.willValidate, 'with readonly attribute');
+  control.removeAttribute('readonly');
+  assert_true(control.i.willValidate, 'without readonly attribute');
 }, 'willValidate');
 
+test(() => {
+  const container = document.getElementById("container");
+  container.innerHTML = '<will-be-defined></will-be-defined>' +
+                        '<will-be-defined disabled></will-be-defined>' +
+                        '<will-be-defined readonly></will-be-defined>' +
+                        '<datalist><will-be-defined></will-be-defined></datalist>' +
+                        '<fieldset disabled><will-be-defined></will-be-defined></fieldset>';
+
+  class WillBeDefined extends HTMLElement {
+    static get formAssociated() { return true; }
+
+    constructor() {
+      super();
+      this.internals_ = this.attachInternals();
+    }
+    get i() { return this.internals_; }
+  }
+  customElements.define('will-be-defined', WillBeDefined);
+  customElements.upgrade(container);
+
+  const controls = document.querySelectorAll('will-be-defined');
+  assert_true(controls[0].i.willValidate, 'default value');
+  assert_false(controls[1].i.willValidate, 'with disabled attribute');
+  assert_false(controls[2].i.willValidate, 'with readOnly attribute');
+  assert_false(controls[3].i.willValidate, 'in datalist');
+  assert_false(controls[4].i.willValidate, 'in disabled fieldset');
+}, 'willValidate after upgrade');
+
+test(() => {
+  const element = new NotFormAssociatedElement();
+  assert_throws_dom('NotSupportedError', () => element.i.willValidate);
+}, "willValidate should throw NotSupportedError if the target element is not a form-associated custom element");
+
 test(() => {
   const control = document.createElement('my-control');
   const validity = control.i.validity;