diff --git a/dom/events/EventStates.h b/dom/events/EventStates.h
index de0f651bd4..eef0ca1002 100644
--- a/dom/events/EventStates.h
+++ b/dom/events/EventStates.h
@@ -291,6 +291,8 @@ private:
 #define NS_EVENT_STATE_PLACEHOLDERSHOWN NS_DEFINE_EVENT_STATE_MACRO(52)
 // Element has focus-within.
 #define NS_EVENT_STATE_FOCUS_WITHIN NS_DEFINE_EVENT_STATE_MACRO(53)
+// Modal <dialog> element
+#define NS_EVENT_STATE_MODAL_DIALOG NS_DEFINE_EVENT_STATE_MACRO(54)
 
 #define DIR_ATTR_STATES (NS_EVENT_STATE_HAS_DIR_ATTR |          \
                          NS_EVENT_STATE_DIR_ATTR_LTR |          \
@@ -309,7 +311,8 @@ private:
 #define ESM_MANAGED_STATES (DIR_ATTR_STATES | NS_EVENT_STATE_ACTIVE | NS_EVENT_STATE_FOCUS |  \
                             NS_EVENT_STATE_HOVER | NS_EVENT_STATE_DRAGOVER |   \
                             NS_EVENT_STATE_URLTARGET | NS_EVENT_STATE_FOCUSRING | \
-                            NS_EVENT_STATE_FULL_SCREEN | NS_EVENT_STATE_FOCUS_WITHIN)
+                            NS_EVENT_STATE_FULL_SCREEN | NS_EVENT_STATE_FOCUS_WITHIN | \
+                            NS_EVENT_STATE_MODAL_DIALOG)
 
 #define INTRINSIC_STATES (~ESM_MANAGED_STATES)
 
diff --git a/dom/html/HTMLDialogElement.cpp b/dom/html/HTMLDialogElement.cpp
index a178e5afea..795b44dc1a 100644
--- a/dom/html/HTMLDialogElement.cpp
+++ b/dom/html/HTMLDialogElement.cpp
@@ -56,6 +56,9 @@ HTMLDialogElement::Close(const mozilla::dom::Optional<nsAString>& aReturnValue)
   ErrorResult ignored;
   SetOpen(false, ignored);
   ignored.SuppressException();
+  
+  RemoveFromTopLayerIfNeeded();
+  
   RefPtr<AsyncEventDispatcher> eventDispatcher =
     new AsyncEventDispatcher(this, NS_LITERAL_STRING("close"), false);
   eventDispatcher->PostDOMEvent();
@@ -72,6 +75,26 @@ HTMLDialogElement::Show()
   ignored.SuppressException();
 }
 
+bool HTMLDialogElement::IsInTopLayer() const {
+  return State().HasState(NS_EVENT_STATE_MODAL_DIALOG);
+}
+
+void HTMLDialogElement::RemoveFromTopLayerIfNeeded() {
+  if (!IsInTopLayer()) {
+    return;
+  }
+  auto predictFunc = [&](Element* element) { return element == this; };
+
+  DebugOnly<Element*> removedElement = OwnerDoc()->TopLayerPop(predictFunc);
+  MOZ_ASSERT(removedElement == this);
+  RemoveStates(NS_EVENT_STATE_MODAL_DIALOG);
+}
+
+void HTMLDialogElement::UnbindFromTree(bool aNullParent) {
+  RemoveFromTopLayerIfNeeded();
+  nsGenericHTMLElement::UnbindFromTree(aNullParent);
+}
+
 void
 HTMLDialogElement::ShowModal(ErrorResult& aError)
 {
@@ -79,6 +102,10 @@ HTMLDialogElement::ShowModal(ErrorResult& aError)
    aError.Throw(NS_ERROR_DOM_INVALID_STATE_ERR);
    return;
   }
+  
+  if (!IsInTopLayer() && OwnerDoc()->TopLayerPush(this)) {
+    AddStates(NS_EVENT_STATE_MODAL_DIALOG);
+  }
 
   SetOpen(true, aError);
   aError.SuppressException();
diff --git a/dom/html/HTMLDialogElement.h b/dom/html/HTMLDialogElement.h
index 0520ef56b0..6386452d91 100644
--- a/dom/html/HTMLDialogElement.h
+++ b/dom/html/HTMLDialogElement.h
@@ -41,10 +41,14 @@ public:
   {
     mReturnValue = aReturnValue;
   }
+  
+  void UnbindFromTree(bool aNullParent = true) /*override*/; 
 
   void Close(const mozilla::dom::Optional<nsAString>& aReturnValue);
   void Show();
   void ShowModal(ErrorResult& aError);
+  
+  bool IsInTopLayer() const;
 
   nsString mReturnValue;
 
@@ -52,6 +56,9 @@ protected:
   virtual ~HTMLDialogElement();
   JSObject* WrapNode(JSContext* aCx,
                      JS::Handle<JSObject*> aGivenProto) override;
+
+private:
+  void RemoveFromTopLayerIfNeeded();
 };
 
 } // namespace dom
diff --git a/layout/style/nsCSSPseudoClassList.h b/layout/style/nsCSSPseudoClassList.h
index 7bdb244c0a..9b1b4355e6 100644
--- a/layout/style/nsCSSPseudoClassList.h
+++ b/layout/style/nsCSSPseudoClassList.h
@@ -190,6 +190,12 @@ CSS_STATE_PSEUDO_CLASS(fullscreen, ":fullscreen",
                        NS_EVENT_STATE_FULL_SCREEN)
 CSS_STATE_PSEUDO_CLASS(mozFullScreen, ":-moz-full-screen", 0, "", NS_EVENT_STATE_FULL_SCREEN)
 
+// Matches if a <dialog> element is modal
+CSS_STATE_PSEUDO_CLASS(mozModalDialog, ":-moz-modal-dialog",
+                       CSS_PSEUDO_CLASS_ENABLED_IN_UA_SHEETS,
+                       "",
+                       NS_EVENT_STATE_MODAL_DIALOG)
+
 // Matches if the element is focused and should show a focus ring
 CSS_STATE_PSEUDO_CLASS(mozFocusRing, ":-moz-focusring", 0, "", NS_EVENT_STATE_FOCUSRING)
 
diff --git a/layout/style/res/html.css b/layout/style/res/html.css
index 93539c0890..7f792a06bc 100644
--- a/layout/style/res/html.css
+++ b/layout/style/res/html.css
@@ -811,14 +811,15 @@ dialog {
   position: absolute;
   inset-inline-start: 0;
   inset-inline-end: 0;
-  color: black;
   margin: auto;
   border-width: initial;
   border-style: solid;
-  border-color: initial;
+  border-color: dimgrey;
   border-image: initial;
+  border-radius: 5px;
   padding: 1em;
-  background: white;
+  color: #222; /*CanvasText*/
+  background: snow; /*Canvas*/
   width: -moz-fit-content;
 }
 
@@ -826,6 +827,15 @@ dialog:not([open]) {
   display: none;
 }
 
+dialog:-moz-modal-dialog {
+  -moz-top-layer: top !important;
+}
+
+/* https://html.spec.whatwg.org/#flow-content-3 */
+dialog::backdrop {
+  background: rgba(0, 0, 0, 0.1);
+}
+
 /* emulation of non-standard HTML <marquee> tag */
 marquee {
   inline-size: -moz-available;