diff --git a/html/semantics/interactive-elements/the-dialog-element/dialog-closedby-start-open.html b/html/semantics/interactive-elements/the-dialog-element/dialog-closedby-start-open.html
index 52432ff1b2c48a..03cafdda6a5ef0 100644
--- a/html/semantics/interactive-elements/the-dialog-element/dialog-closedby-start-open.html
+++ b/html/semantics/interactive-elements/the-dialog-element/dialog-closedby-start-open.html
@@ -16,12 +16,16 @@
 promise_test(async (t) => {
   const dialog = document.querySelector('dialog#test1');
   assert_true(dialog.open);
+  assert_true(dialog.matches(':open'));
   await new test_driver.send_keys(document.documentElement,ESC);
   assert_false(dialog.open);
+  assert_false(dialog.matches(':open'));
   dialog.showModal();
   assert_true(dialog.open);
+  assert_true(dialog.matches(':open'));
   await new test_driver.send_keys(document.documentElement,ESC);
   assert_false(dialog.open);
+  assert_false(dialog.matches(':open'));
 }, `Dialogs that start open and have closedby should still function`);
 </script>
 
@@ -48,7 +52,9 @@
     });
   });
   assert_true(dialog.open);
+  assert_true(dialog.matches(':open'));
   await new test_driver.send_keys(document.documentElement,ESC);
   assert_false(dialog.open,'ESC should still work');
+  assert_false(dialog.matches(':open'));
 }, `Opening and closing a dialog during the dialog focus fixup should still leave closedby functional`);
 </script>
diff --git a/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html b/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html
index 65902ed2d5ffc2..374a9803883a99 100644
--- a/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html
+++ b/html/semantics/interactive-elements/the-dialog-element/dialog-closedby.html
@@ -27,18 +27,21 @@
 <script>
   function openDialog(dialog,modal) {
     assert_false(dialog.open);
+    assert_false(dialog.matches(':open'));
     if (modal) {
       dialog.showModal();
     } else {
       dialog.show();
     }
     assert_true(dialog.open);
+    assert_true(dialog.matches(':open'));
     assert_equals(dialog.matches(':modal'),modal);
   }
   function runTest(dialog) {
     for(modal of [false,true]) {
       promise_test(async (t) => {
         assert_false(dialog.open);
+        assert_false(dialog.matches(':open'));
         t.add_cleanup(() => dialog.close());
         // Try hitting ESC
         openDialog(dialog,modal);
@@ -46,11 +49,13 @@
         const ESC = '\uE00C';
         await new test_driver.send_keys(document.documentElement,ESC);
         const respondsToEsc = !dialog.open;
+        const respondsToEsc2 = !dialog.matches(':open');
         dialog.close();
         // Try clicking outside
         openDialog(dialog,modal);
         await clickOn(outside);
         const respondsToLightDismiss = !dialog.open;
+        const respondsToLightDismiss2 = !dialog.matches(':open');
         dialog.close();
         // See if expectations match
         let expectedReflectionWhileOpen = dialog.dataset.behavior;
@@ -58,15 +63,21 @@
         switch (dialog.dataset.behavior) {
           case 'any':
             assert_true(respondsToEsc,'Dialog should respond to ESC');
+            assert_true(respondsToEsc2,'Dialog should respond to ESC (:open)');
             assert_true(respondsToLightDismiss,'Dialog should respond to light dismiss');
+            assert_true(respondsToLightDismiss2,'Dialog should respond to light dismiss (:open)');
             break;
           case 'closerequest':
             assert_true(respondsToEsc,'Dialog should respond to ESC');
+            assert_true(respondsToEsc2,'Dialog should respond to ESC (:open)');
             assert_false(respondsToLightDismiss,'Dialog should NOT respond to light dismiss');
+            assert_false(respondsToLightDismiss2,'Dialog should NOT respond to light dismiss (:open)');
             break;
           case 'none':
             assert_false(respondsToEsc,'Dialog should NOT respond to ESC');
+            assert_false(respondsToEsc2,'Dialog should NOT respond to ESC (:open)');
             assert_false(respondsToLightDismiss,'Dialog should NOT respond to light dismiss');
+            assert_false(respondsToLightDismiss2,'Dialog should NOT respond to light dismiss (:open)');
             break;
           case 'auto':
             if (modal) {
diff --git a/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.html b/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.html
index 3790926dda42c1..f63bc91ab475e3 100644
--- a/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.html
+++ b/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.html
@@ -11,15 +11,25 @@
 
 <script>
 const dialog = document.querySelector('dialog');
-function openDialog(modal) {
+function openDialog(openMethod) {
   assert_false(dialog.open);
-  if (modal) {
-    dialog.showModal();
-  } else {
-    dialog.show();
+  assert_false(dialog.matches(':open'));
+  switch (openMethod) {
+    case 'modeless':
+      dialog.show();
+      break;
+    case 'modal':
+      dialog.showModal();
+      break;
+    case 'open':
+      dialog.open = true;
+      break;
+    default:
+      assert_unreached('Unknown open method');
   }
   assert_true(dialog.open);
-  assert_equals(dialog.matches(':modal'),modal);
+  assert_true(dialog.matches(':open'));
+  assert_equals(dialog.matches(':modal'),openMethod === 'modal');
 }
 function getSignal(t) {
   const controller = new AbortController();
@@ -43,14 +53,15 @@
   return getSignal(t);
 }
 
-[false,true].forEach(modal => {
+['modeless','modal','open'].forEach(openMethod => {
   [null,'any','closedrequest','none'].forEach(closedby => {
-    const testDescription = `for ${modal ? "modal" : "modeless"} dialog with closedby=${closedby}`;
+    const testDescription = `for ${openMethod} dialog with closedby=${closedby}`;
     promise_test(async (t) => {
       await setup(t,closedby);
-      openDialog(modal);
+      openDialog(openMethod);
       dialog.requestClose();
       assert_false(dialog.open);
+      assert_false(dialog.matches(':open'));
     },`requestClose basic behavior ${testDescription}`);
 
     promise_test(async (t) => {
@@ -58,10 +69,11 @@
       let events = [];
       dialog.addEventListener('cancel',() => events.push('cancel'),{signal});
       dialog.addEventListener('close',() => events.push('close'),{signal});
-      openDialog(modal);
+      openDialog(openMethod);
       assert_array_equals(events,[]);
       dialog.requestClose();
       assert_false(dialog.open);
+      assert_false(dialog.matches(':open'));
       assert_array_equals(events,['cancel'],'close is scheduled');
       await new Promise(resolve => requestAnimationFrame(resolve));
       assert_array_equals(events,['cancel','close']);
@@ -72,14 +84,15 @@
       let events = [];
       dialog.addEventListener('cancel',() => events.push('cancel'),{signal});
       dialog.addEventListener('close',() => events.push('close'),{signal});
-      openDialog(modal);
+      openDialog(openMethod);
       dialog.setAttribute('closedby',closedby);
       assert_array_equals(events,[]);
       dialog.requestClose();
       assert_false(dialog.open,'Adding closedby after dialog is open');
+      assert_false(dialog.matches(':open'));
       assert_array_equals(events,['cancel']);
       events=[];
-      openDialog(modal);
+      openDialog(openMethod);
       dialog.removeAttribute('closedby');
       assert_array_equals(events,[]);
       dialog.requestClose();
@@ -96,32 +109,36 @@
             e.preventDefault();
           }
         },{signal});
-        openDialog(modal);
+        openDialog(openMethod);
         dialog.requestClose();
         assert_true(dialog.open,'cancel event was cancelled - dialog shouldn\'t close');
+        assert_true(dialog.matches(':open'));
         shouldPreventDefault = false;
         dialog.requestClose();
         assert_false(dialog.open,'cancel event was not cancelled - dialog should now close');
+        assert_false(dialog.matches(':open'));
       },`requestClose can be cancelled ${testDescription}`);
 
       promise_test(async (t) => {
         const signal = await setup(t,closedby);
         dialog.addEventListener('cancel',(e) => e.preventDefault(),{signal});
-        openDialog(modal);
+        openDialog(openMethod);
         // No user activation here.
         dialog.requestClose();
         dialog.requestClose();
         dialog.requestClose();
         assert_true(dialog.open,'cancel event was cancelled - dialog shouldn\'t close');
+        assert_true(dialog.matches(':open'));
       },`requestClose avoids abuse prevention logic ${testDescription}`);
 
       promise_test(async (t) => {
         await setup(t,closedby);
-        openDialog(modal);
+        openDialog(openMethod);
         assert_equals(dialog.returnValue,'','Return value starts out empty');
         const returnValue = 'The return value';
         dialog.requestClose(returnValue);
         assert_false(dialog.open);
+        assert_false(dialog.matches(':open'));
         assert_equals(dialog.returnValue,returnValue,'Return value should be set');
         dialog.show();
         dialog.close();
@@ -134,11 +151,12 @@
       promise_test(async (t) => {
         await setup(t,closedby);
         dialog.addEventListener('cancel',(e) => e.preventDefault(),{once:true});
-        openDialog(modal);
+        openDialog(openMethod);
         dialog.returnValue = 'foo';
         assert_equals(dialog.returnValue,'foo');
         dialog.requestClose('This should not get saved');
         assert_true(dialog.open,'cancelled');
+        assert_true(dialog.matches(':open'));
         assert_equals(dialog.returnValue,'foo','Return value should not be changed');
       },`requestClose(returnValue) doesn't change returnvalue when cancelled ${testDescription}`);
     }