diff --git a/html/semantics/popups/popup-animation-corner-cases.tentative.html b/html/semantics/popups/popup-animation-corner-cases.tentative.html
index 917f0d900d1502..c463139b3c64ec 100644
--- a/html/semantics/popups/popup-animation-corner-cases.tentative.html
+++ b/html/semantics/popups/popup-animation-corner-cases.tentative.html
@@ -45,7 +45,10 @@
   const {popUp, descendent} = createPopUp(t,'animation');
   assert_false(isElementVisible(popUp));
   assert_equals(descendent.parentElement.parentElement,popUp);
+  assert_true(popUp.matches(':closed'));
+  assert_false(popUp.matches(':open'));
   popUp.showPopUp();
+  assert_false(popUp.matches(':closed'));
   assert_true(popUp.matches(':open'));
   assert_true(isElementVisible(popUp));
   assert_equals(popUp.getAnimations({subtree: true}).length,0);
@@ -53,6 +56,7 @@
   const animations = popUp.getAnimations({subtree: true});
   assert_equals(animations.length,2,'There should be two animations running');
   assert_false(popUp.matches(':open'),'popUp should not match :open as soon as hidden');
+  assert_false(popUp.matches(':closed'),'popUp should not match :closed until animations complete');
   assert_true(isElementVisible(popUp),'but animations should keep the popUp visible');
   assert_true(isElementVisible(descendent),'The descendent should also be visible');
   await waitForRender();
@@ -61,6 +65,7 @@
   assert_true(isElementVisible(popUp),'PopUp should still be visible due to animation');
   animations.forEach(animation => animation.finish()); // Force the animations to finish
   await waitForRender(); // Wait one frame
+  assert_true(popUp.matches(':closed'),'The pop up should now match :closed');
   assert_false(popUp.matches(':open'),'The pop up still shouldn\'t match :open');
   assert_false(isElementVisible(popUp),'The pop up should now be invisible');
   assert_false(isElementVisible(descendent),'The descendent should also be invisible');
@@ -82,6 +87,7 @@
   // Then hide the popUp.
   popUp.hidePopUp();
   assert_false(popUp.matches(':open'),'pop up should not match :open as soon as hidden');
+  assert_true(popUp.matches(':closed'),'pop up should match :closed immediately');
   assert_equals(popUp.getAnimations({subtree: true}).length,2,'animations should still be running');
   await waitForRender();
   assert_equals(popUp.getAnimations({subtree: true}).length,2,'animations should still be running');
@@ -137,6 +143,27 @@
   assert_false(isElementVisible(popUp),'Even if hide event is cancelled, the popup still closes');
 },'hide event cannot be cancelled');
 
+promise_test(async (t) => {
+  const {popUp, descendent} = createPopUp(t,'animation');
+  assert_false(isElementVisible(popUp));
+  popUp.showPopUp();
+  assert_false(popUp.matches(':closed'));
+  assert_true(popUp.matches(':open'));
+  assert_true(isElementVisible(popUp));
+  assert_equals(popUp.getAnimations({subtree: true}).length,0);
+  popUp.popUp = 'manual';
+  const animations = popUp.getAnimations({subtree: true});
+  assert_equals(animations.length,2,'There should be two animations running');
+  assert_false(popUp.matches(':open'),'popUp should not match :open as soon as hidden');
+  assert_false(popUp.matches(':closed'),'popUp should not match :closed until animations complete');
+  assert_true(isElementVisible(popUp),'but animations should keep the popUp visible');
+  animations.forEach(animation => animation.finish()); // Force the animations to finish
+  await waitForRender(); // Wait one frame
+  assert_true(popUp.matches(':closed'),'The pop up should now match :closed');
+  assert_false(popUp.matches(':open'),'The pop up still shouldn\'t match :open');
+  assert_false(isElementVisible(popUp),'The pop up should now be invisible');
+},'Closing animations are triggered by changing the pop-up type');
+
 promise_test(async (t) => {
   const {popUp, descendent} = createPopUp(t,'');
   popUp.showPopUp();
diff --git a/html/semantics/popups/popup-attribute-basic.tentative.html b/html/semantics/popups/popup-attribute-basic.tentative.html
index c20499c3f30fd9..1edcb491cf46f0 100644
--- a/html/semantics/popups/popup-attribute-basic.tentative.html
+++ b/html/semantics/popups/popup-attribute-basic.tentative.html
@@ -48,9 +48,11 @@
     if (isVisible) {
       assert_not_equals(window.getComputedStyle(popUp).display,'none');
       assert_equals(popUp.matches(':open'),isPopUp,`${message}: Visible pop-ups should match :open`);
+      assert_false(popUp.matches(':closed'),`${message}: Visible pop-ups and *all* non-pop-ups should *not* match :closed`);
     } else {
       assert_equals(window.getComputedStyle(popUp).display,'none',`${message}: Non-showing pop-ups should have display:none`);
       assert_false(popUp.matches(':open'),`${message}: Non-showing pop-ups should *not* match :open`);
+      assert_true(popUp.matches(':closed'),`${message}: Non-showing pop-ups should match :closed`);
     }
   }
   function assertIsFunctionalPopUp(popUp) {
@@ -90,7 +92,7 @@
   });
 
   // Then loop through all HTML5 elements that render a box by default:
-  let elementsThatDontRender = ['audio','base','br','datalist','dialog','embed','head','link','meta','noscript','param','rp','script','style','template','title','wbr'];
+  let elementsThatDontRender = ['audio','base','br','datalist','dialog','embed','head','link','meta','noscript','param','rp','script','slot','style','template','title','wbr'];
   const elements = HTML5_ELEMENTS.filter(el => !elementsThatDontRender.includes(el));
   elements.forEach(tag => {
     test((t) => {
@@ -99,7 +101,13 @@
       document.body.appendChild(element);
       t.add_cleanup(() => element.remove());
       assertIsFunctionalPopUp(element);
-    }, `A <${tag}> element should behave as a pop-up.`);
+    }, `A <${tag} popup> element should behave as a pop-up.`);
+    test((t) => {
+      const element = document.createElement(tag);
+      document.body.appendChild(element);
+      t.add_cleanup(() => element.remove());
+      assertNotAPopUp(element);
+    }, `A <${tag}> element should *not* behave as a pop-up.`);
   });
 
   function createPopUp(t) {
@@ -326,6 +334,7 @@
     assert_false(isElementVisible(popUp));
     popUp.showPopUp();
     assert_true(popUp.matches(':open'));
+    assert_false(popUp.matches(':closed'));
     assert_true(getComputedStyle(popUp).opacity < 0.1,'Animations should start on show');
     assert_throws_dom("InvalidStateError",() => popUp.showPopUp(),'Calling showPopUp on a popup that is in the process of animating show should throw InvalidStateError');
     await finishAnimations(popUp);
@@ -333,10 +342,12 @@
     assert_true(isElementVisible(popUp));
     popUp.hidePopUp();
     assert_false(popUp.matches(':open'));
+    assert_false(popUp.matches(':closed'),'Not :closed until animations finish');
     assert_true(getComputedStyle(popUp).opacity > 0.9,'Animations should start on hide');
     assert_throws_dom("InvalidStateError",() => popUp.hidePopUp(),'Calling hidePopUp on a popup that is in the process of animating hide should throw InvalidStateError');
     popUp.showPopUp(); // But showPopUp should still be ok.
     popUp.hidePopUp(); // Clean up
     await finishAnimations(popUp);
+    assert_true(popUp.matches(':closed'),':closed should match once animations finish');
   },'Exceptions are thrown even when show/hide are animated')
 </script>