diff --git a/dom/html/HTMLDialogElement.cpp b/dom/html/HTMLDialogElement.cpp
index 5af7d6cf75387..c80d4e6298010 100644
--- a/dom/html/HTMLDialogElement.cpp
+++ b/dom/html/HTMLDialogElement.cpp
@@ -71,7 +71,11 @@ void HTMLDialogElement::Close(
 
 void HTMLDialogElement::Show(ErrorResult& aError) {
   if (Open()) {
-    return;
+    if (!IsInTopLayer()) {
+      return;
+    }
+    return aError.ThrowInvalidStateError(
+        "Cannot call show() on an open modal dialog.");
   }
 
   if (IsPopoverOpen()) {
@@ -121,13 +125,16 @@ void HTMLDialogElement::UnbindFromTree(bool aNullParent) {
 }
 
 void HTMLDialogElement::ShowModal(ErrorResult& aError) {
-  if (!IsInComposedDoc()) {
-    return aError.ThrowInvalidStateError("Dialog element is not connected");
-  }
-
   if (Open()) {
+    if (IsInTopLayer()) {
+      return;
+    }
     return aError.ThrowInvalidStateError(
-        "Dialog element already has an 'open' attribute");
+        "Cannot call showModal() on an open non-modal dialog.");
+  }
+
+  if (!IsInComposedDoc()) {
+    return aError.ThrowInvalidStateError("Dialog element is not connected");
   }
 
   if (IsPopoverOpen()) {
diff --git a/testing/web-platform/meta/html/semantics/interactive-elements/the-dialog-element/dialog-no-throw-requested-state.html.ini b/testing/web-platform/meta/html/semantics/interactive-elements/the-dialog-element/dialog-no-throw-requested-state.html.ini
deleted file mode 100644
index d1af0a488be0f..0000000000000
--- a/testing/web-platform/meta/html/semantics/interactive-elements/the-dialog-element/dialog-no-throw-requested-state.html.ini
+++ /dev/null
@@ -1,3 +0,0 @@
-[dialog-no-throw-requested-state.html]
-  [dialog-no-throw-requested-state]
-    expected: FAIL