diff --git a/html/semantics/popups/popup-initiallyopen-2.tentative.html b/html/semantics/popups/popup-defaultopen-2.tentative.html
similarity index 80%
rename from html/semantics/popups/popup-initiallyopen-2.tentative.html
rename to html/semantics/popups/popup-defaultopen-2.tentative.html
index a55d33d3ea6e95..1e54e469ae8410 100644
--- a/html/semantics/popups/popup-initiallyopen-2.tentative.html
+++ b/html/semantics/popups/popup-defaultopen-2.tentative.html
@@ -5,9 +5,9 @@
 <script src="/resources/testharness.js"></script>
 <script src="/resources/testharnessreport.js"></script>
 
-<div popup=popup id=p1 initiallyopen>
+<div popup=popup id=p1 defaultopen>
   Outer popup
-  <div popup=popup id=p2 initiallyopen>
+  <div popup=popup id=p2 defaultopen>
     Inner popup
   </div>
 </div>
@@ -18,7 +18,7 @@
       test(function(){
         assert_true(p1.matches(':popup-open'),'The first (outermost) popup should be the one that opens in this case');
         assert_false(p2.matches(':popup-open'),'The inner popup should not be open');
-      }, "The initiallyopen attribute should cause only the first popup to open");
+      }, "The defaultopen attribute should cause only the first popup to open");
     });
   });
 </script>
diff --git a/html/semantics/popups/popup-initiallyopen-display-ref.tentative.html b/html/semantics/popups/popup-defaultopen-display-ref.tentative.html
similarity index 100%
rename from html/semantics/popups/popup-initiallyopen-display-ref.tentative.html
rename to html/semantics/popups/popup-defaultopen-display-ref.tentative.html
diff --git a/html/semantics/popups/popup-initiallyopen-display.tentative.html b/html/semantics/popups/popup-defaultopen-display.tentative.html
similarity index 57%
rename from html/semantics/popups/popup-initiallyopen-display.tentative.html
rename to html/semantics/popups/popup-defaultopen-display.tentative.html
index baefeca9fff11f..179e07189b31ec 100644
--- a/html/semantics/popups/popup-initiallyopen-display.tentative.html
+++ b/html/semantics/popups/popup-defaultopen-display.tentative.html
@@ -2,10 +2,10 @@
 <meta charset="utf-8">
 <link rel=author href="mailto:masonf@chromium.org">
 <link rel=help href="https://open-ui.org/components/popup.research.explainer">
-<link rel=match href="popup-initiallyopen-display-ref.tentative.html">
+<link rel=match href="popup-defaultopen-display-ref.tentative.html">
 
-<div popup=popup id=p1 initiallyopen>This is a popup, which should be open upon load</div>
-<div popup=popup id=p2 initiallyopen>This is a second popup with initiallyopen, which should NOT be open upon load</div>
+<div popup=popup id=p1 defaultopen>This is a popup, which should be open upon load</div>
+<div popup=popup id=p2 defaultopen>This is a second popup with defaultopen, which should NOT be open upon load</div>
 
 <style>
   [popup] {
diff --git a/html/semantics/popups/popup-defaultopen.tentative.html b/html/semantics/popups/popup-defaultopen.tentative.html
new file mode 100644
index 00000000000000..50bf825d8f2672
--- /dev/null
+++ b/html/semantics/popups/popup-defaultopen.tentative.html
@@ -0,0 +1,46 @@
+<!DOCTYPE html>
+<meta charset="utf-8">
+<link rel=author href="mailto:masonf@chromium.org">
+<link rel=help href="https://open-ui.org/components/popup.research.explainer">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<div popup=popup id=p1 defaultopen>This is a popup, which should be open upon load</div>
+<script></script> <!-- Possibly yield the parser, just to double-check -->
+<div popup=popup id=p2 defaultopen>This is a second popup with defaultopen, which should NOT be open upon load</div>
+<div popup=hint id=p2b defaultopen>This is a hint popup with defaultopen, which should NOT be open upon load</div>
+<div popup=popup id=p3>Also not visible</div>
+
+<div popup=async id=p4 defaultopen>This is an async popup with defaultopen, which should be open upon load</div>
+<div popup=async id=p5 defaultopen>This is an async popup with defaultopen, which should be open upon load</div>
+
+<script>
+  requestAnimationFrame(() => {
+    requestAnimationFrame(() => {
+      test(function(){
+        assert_true(p1.matches(':popup-open'),'defaultopen should open the popup');
+        assert_true(p1.hasAttribute('defaultopen'));
+        assert_true(p1.defaultOpen,'defaultopen should be reflected in the IDL attribute');
+        assert_false(p2.matches(':popup-open'), 'Only the first popup with defaultopen should be open on load');
+        assert_true(p2.hasAttribute('defaultopen'),'defaultopen should be present/true, even if not opened');
+        assert_true(p2.defaultOpen,'defaultopen should be present/true, even if not opened');
+
+        assert_false(p2b.matches(':popup-open'),'Only the first popup/hint with defaultopen should be open on load');
+        assert_true(p4.matches(':popup-open'),'defaultopen should open all async popups');
+        assert_true(p5.matches(':popup-open'),'defaultopen should open all async popups');
+
+        assert_false(p3.matches(':popup-open'));
+        p3.setAttribute('defaultopen','');
+        assert_false(p3.matches(':popup-open'), 'Changing defaultopen should not affect open status');
+        assert_true(p3.hasAttribute('defaultopen'));
+        assert_true(p3.defaultOpen,'defaultopen should still reflect to IDL');
+
+        p1.removeAttribute('defaultopen');
+        assert_true(p1.matches(':popup-open'),'removing defaultopen should not close the popup');
+        assert_false(p1.hasAttribute('defaultopen'),'...but it should reflect to IDL');
+
+        p1.hidePopup();
+      }, "The defaultopen attribute should affect page load only");
+    });
+  });
+</script>
diff --git a/html/semantics/popups/popup-initiallyopen.tentative.html b/html/semantics/popups/popup-initiallyopen.tentative.html
deleted file mode 100644
index 9396e014afd182..00000000000000
--- a/html/semantics/popups/popup-initiallyopen.tentative.html
+++ /dev/null
@@ -1,46 +0,0 @@
-<!DOCTYPE html>
-<meta charset="utf-8">
-<link rel=author href="mailto:masonf@chromium.org">
-<link rel=help href="https://open-ui.org/components/popup.research.explainer">
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
-
-<div popup=popup id=p1 initiallyopen>This is a popup, which should be open upon load</div>
-<script></script> <!-- Possibly yield the parser, just to double-check -->
-<div popup=popup id=p2 initiallyopen>This is a second popup with initiallyopen, which should NOT be open upon load</div>
-<div popup=hint id=p2b initiallyopen>This is a hint popup with initiallyopen, which should NOT be open upon load</div>
-<div popup=popup id=p3>Also not visible</div>
-
-<div popup=async id=p4 initiallyopen>This is an async popup with initiallyopen, which should be open upon load</div>
-<div popup=async id=p5 initiallyopen>This is an async popup with initiallyopen, which should be open upon load</div>
-
-<script>
-  requestAnimationFrame(() => {
-    requestAnimationFrame(() => {
-      test(function(){
-        assert_true(p1.matches(':popup-open'),'initiallyopen should open the popup');
-        assert_true(p1.hasAttribute('initiallyopen'));
-        assert_true(p1.initiallyOpen,'initiallyopen should be reflected in the IDL attribute');
-        assert_false(p2.matches(':popup-open'), 'Only the first popup with initiallyopen should be open on load');
-        assert_true(p2.hasAttribute('initiallyopen'),'initiallyopen should be present/true, even if not opened');
-        assert_true(p2.initiallyOpen,'initiallyopen should be present/true, even if not opened');
-
-        assert_false(p2b.matches(':popup-open'),'Only the first popup/hint with initiallyopen should be open on load');
-        assert_true(p4.matches(':popup-open'),'initiallyopen should open all async popups');
-        assert_true(p5.matches(':popup-open'),'initiallyopen should open all async popups');
-
-        assert_false(p3.matches(':popup-open'));
-        p3.setAttribute('initiallyopen','');
-        assert_false(p3.matches(':popup-open'), 'Changing initiallyopen should not affect open status');
-        assert_true(p3.hasAttribute('initiallyopen'));
-        assert_true(p3.initiallyOpen,'initiallyopen should still reflect to IDL');
-
-        p1.removeAttribute('initiallyopen');
-        assert_true(p1.matches(':popup-open'),'removing initiallyopen should not close the popup');
-        assert_false(p1.hasAttribute('initiallyopen'),'...but it should reflect to IDL');
-
-        p1.hidePopup();
-      }, "The initiallyopen attribute should affect page load only");
-    });
-  });
-</script>
