diff --git a/html/semantics/popups/popup-animated-hide-cleanup.tentative.html b/html/semantics/popups/popup-animated-hide-cleanup.tentative.html
index 762458ea669dda..0a954519a4461b 100644
--- a/html/semantics/popups/popup-animated-hide-cleanup.tentative.html
+++ b/html/semantics/popups/popup-animated-hide-cleanup.tentative.html
@@ -47,7 +47,7 @@
 }
 function addPopup(classname) {
   const popup = document.createElement('div');
-  popup.popup = 'auto';
+  popup.popUp = 'auto';
   popup.classList = classname;
   popup.textContent = 'This is a popup';
   document.body.appendChild(popup);
diff --git a/html/semantics/popups/popup-attribute-basic.tentative.html b/html/semantics/popups/popup-attribute-basic.tentative.html
index 22742a3a6edcd1..45090c5d26c00c 100644
--- a/html/semantics/popups/popup-attribute-basic.tentative.html
+++ b/html/semantics/popups/popup-attribute-basic.tentative.html
@@ -79,43 +79,43 @@
     // Setting IDL to something sets the content attribute to exactly that, always.
     // GETTING the IDL only gets valid values.
     const popup = createPopup(t);
-    assert_equals(popup.popup,'auto');
+    assert_equals(popup.popUp,'auto');
     popup.setAttribute('popup','hint');
-    assert_equals(popup.popup,'hint');
+    assert_equals(popup.popUp,'hint');
     popup.setAttribute('popup','HiNt');
-    assert_equals(popup.popup,'hint','Case is normalized in IDL');
+    assert_equals(popup.popUp,'hint','Case is normalized in IDL');
     assert_equals(popup.getAttribute('popup'),'HiNt','Case is *not* normalized/changed in the content attribute');
-    popup.popup='hInT';
-    assert_equals(popup.popup,'hint','Case is normalized in IDL');
+    popup.popUp='hInT';
+    assert_equals(popup.popUp,'hint','Case is normalized in IDL');
     assert_equals(popup.getAttribute('popup'),'hInT','Value set from IDL is propagated exactly to the content attribute');
     popup.setAttribute('popup','invalid');
-    assert_equals(popup.popup,null,'Invalid values should reflect as null');
+    assert_equals(popup.popUp,null,'Invalid values should reflect as null');
     popup.removeAttribute('popup');
-    assert_equals(popup.popup,null,'No value should reflect as null');
-    popup.popup='hint';
+    assert_equals(popup.popUp,null,'No value should reflect as null');
+    popup.popUp='hint';
     assert_equals(popup.getAttribute('popup'),'hint');
-    popup.popup='auto';
+    popup.popUp='auto';
     assert_equals(popup.getAttribute('popup'),'auto');
-    popup.popup='';
+    popup.popUp='';
     assert_equals(popup.getAttribute('popup'),'');
-    assert_equals(popup.popup,'auto');
-    popup.popup='AuTo';
+    assert_equals(popup.popUp,'auto');
+    popup.popUp='AuTo';
     assert_equals(popup.getAttribute('popup'),'AuTo');
-    assert_equals(popup.popup,'auto');
-    popup.popup='invalid';
+    assert_equals(popup.popUp,'auto');
+    popup.popUp='invalid';
     assert_equals(popup.getAttribute('popup'),'invalid','IDL setter allows any value');
-    assert_equals(popup.popup,null,'but IDL getter does not re-reflect invalid values');
-    popup.popup='';
+    assert_equals(popup.popUp,null,'but IDL getter does not re-reflect invalid values');
+    popup.popUp='';
     assert_equals(popup.getAttribute('popup'),'','IDL setter propagates exactly');
-    assert_equals(popup.popup,'auto','Empty should map to auto in IDL');
-    popup.popup='auto';
-    popup.popup=null;
+    assert_equals(popup.popUp,'auto','Empty should map to auto in IDL');
+    popup.popUp='auto';
+    popup.popUp=null;
     assert_equals(popup.getAttribute('popup'),null,'Setting null for the IDL property should remove the content attribute');
-    assert_equals(popup.popup,null,'Null returns null');
-    popup.popup='auto';
-    popup.popup=undefined;
+    assert_equals(popup.popUp,null,'Null returns null');
+    popup.popUp='auto';
+    popup.popUp=undefined;
     assert_equals(popup.getAttribute('popup'),null,'Setting undefined for the IDL property should remove the content attribute');
-    assert_equals(popup.popup,null,'undefined returns null');
+    assert_equals(popup.popUp,null,'undefined returns null');
   },'IDL attribute reflection');
 
   test((t) => {
@@ -129,11 +129,11 @@
     popup.setAttribute('PoPuP','AuTo');
     assertIsFunctionalPopup(popup);
     // Via IDL also
-    popup.popup = 'auto';
+    popup.popUp = 'auto';
     assertIsFunctionalPopup(popup);
-    popup.popup = 'aUtO';
+    popup.popUp = 'aUtO';
     assertIsFunctionalPopup(popup);
-    popup.popup = 'invalid';
+    popup.popUp = 'invalid';
     assertNotAPopup(popup);
   },'Popup attribute value should be case insensitive');
 
@@ -144,9 +144,9 @@
     assertIsFunctionalPopup(popup);
     popup.setAttribute('popup','invalid'); // Change popup type to something invalid
     assertNotAPopup(popup);
-    popup.popup = 'hint'; // Change popup type via IDL
+    popup.popUp = 'hint'; // Change popup type via IDL
     assertIsFunctionalPopup(popup);
-    popup.popup = 'invalid'; // Make invalid via IDL
+    popup.popUp = 'invalid'; // Make invalid via IDL
     assertNotAPopup(popup);
   },'Changing attribute values for popup should work');
 
diff --git a/html/semantics/popups/popup-focus.tentative.html b/html/semantics/popups/popup-focus.tentative.html
index 85f328f0e10345..b1d9f475944ccf 100644
--- a/html/semantics/popups/popup-focus.tentative.html
+++ b/html/semantics/popups/popup-focus.tentative.html
@@ -154,11 +154,11 @@
       priorFocus.focus();
       popup.showPopup();
       assert_equals(document.activeElement, expectedFocusedElement, `${testName} activated by popup.showPopup()`);
-      assert_equals(popup.popup, 'auto', 'All popups in this test should start as popup=auto');
-      popup.popup = 'hint';
+      assert_equals(popup.popUp, 'auto', 'All popups in this test should start as popup=auto');
+      popup.popUp = 'hint';
       assert_false(popup.matches(':top-layer'), 'Changing the popup type should hide the popup');
       assert_equals(document.activeElement, priorFocus, 'prior element should get focus when the type is changed');
-      popup.popup = 'auto';
+      popup.popUp = 'auto';
 
       // Remove from the document:
       priorFocus.focus();
diff --git a/html/semantics/popups/popup-invoking-attribute.tentative.html b/html/semantics/popups/popup-invoking-attribute.tentative.html
index 4a40768647156a..58dbb8e463bb45 100644
--- a/html/semantics/popups/popup-invoking-attribute.tentative.html
+++ b/html/semantics/popups/popup-invoking-attribute.tentative.html
@@ -117,10 +117,10 @@
       s_set.forEach(s => {
         h_set.forEach(h => {
           promise_test(async test => {
-            const popup1 = Object.assign(document.createElement('div'),{popup: type, id: 'popup-1'});
-            const popup2 = Object.assign(document.createElement('div'),{popup: type, id: 'popup-2'});
-            assert_equals(popup1.popup,type);
-            assert_equals(popup2.popup,type);
+            const popup1 = Object.assign(document.createElement('div'),{popUp: type, id: 'popup-1'});
+            const popup2 = Object.assign(document.createElement('div'),{popUp: type, id: 'popup-2'});
+            assert_equals(popup1.popUp,type);
+            assert_equals(popup2.popUp,type);
             assert_not_equals(popup1.id,popup2.id);
             const invoker = testcase.makeElement(test);
             if (t) invoker.setAttribute('togglepopup',t===1 ? popup1.id : popup2.id);
diff --git a/html/semantics/popups/popup-top-layer-interactions.tentative.html b/html/semantics/popups/popup-top-layer-interactions.tentative.html
index c8f9ddf658f8f5..badf581d8941cf 100644
--- a/html/semantics/popups/popup-top-layer-interactions.tentative.html
+++ b/html/semantics/popups/popup-top-layer-interactions.tentative.html
@@ -11,22 +11,22 @@
 <body>
 <script>
 const types = Object.freeze({
-  popup: Symbol("Popup API"),
+  popUp: Symbol("Popup API"),
   modalDialog: Symbol("Modal Dialog"),
   fullscreen: Symbol("Fullscreen Element"),
 });
 const examples = [
   {
-    type: types.popup,
-    closes: [types.popup],
-    createElement: () => Object.assign(document.createElement('div'), {popup: 'auto'}),
+    type: types.popUp,
+    closes: [types.popUp],
+    createElement: () => Object.assign(document.createElement('div'), {popUp: 'auto'}),
     trigger: function() {this.element.showPopup()},
     close: function() {this.element.hidePopup()},
     isTopLayer: function() {return this.element.matches(':top-layer')},
   },
   {
     type: types.modalDialog,
-    closes: [types.popup],
+    closes: [types.popUp],
     createElement: () => document.createElement('dialog'),
     trigger: function() {this.element.showModal();this.showing=true;},
     close: function() {this.element.close();this.showing=false;},
@@ -34,7 +34,7 @@
   },
   {
     type: types.fullscreen,
-    closes: [types.popup, types.fullscreen],
+    closes: [types.popUp, types.fullscreen],
     createElement: () => document.createElement('div'),
     trigger: async function(visibleElement) {assert_false(this.isTopLayer());await bless(visibleElement);await this.element.requestFullscreen();},
     close: function() {assert_equals(this.element,document.fullscreenElement); document.exitFullscreen();},