diff --git a/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
index ed52f78a8872..eb2270b861c7 100644
--- a/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
+++ b/testing/web-platform/tests/html/semantics/interactive-elements/the-dialog-element/dialog-requestclose.tentative.html
@@ -41,31 +41,132 @@
   assert_false(dialog.open,'Without user activation, requestClose can\'t be cancelled');
 },`requestClose requires user activation in order to be cancelable`);
 
+async function setup(t,closedby) {
+  t.add_cleanup(() => {
+    dialog.close();
+    dialog.removeAttribute('closedby');
+    dialog.returnValue = '';
+  });
+  assert_false(dialog.hasAttribute('closedby'));
+  if (closedby) {
+    dialog.setAttribute('closedby',closedby);
+  }
+  // Be sure any pending close events get fired.
+  await new Promise(resolve => requestAnimationFrame(resolve));
+  return getSignal(t);
+}
+
 [false,true].forEach(modal => {
-  promise_test(async (t) => {
-    t.add_cleanup(() => dialog.close());
-    openDialog(modal);
-    dialog.requestClose();
-    assert_false(dialog.open);
-  },`${modal ? "Modal:" : "Non-modal:"} requestClose closes the dialog`);
+  [null,'any','closedrequest','none'].forEach(closedby => {
+    const testDescription = `for ${modal ? "modal" : "modeless"} dialog with closedby=${closedby}`;
+    promise_test(async (t) => {
+      await setup(t,closedby);
+      openDialog(modal);
+      if (dialog.closedBy != "none") {
+        dialog.requestClose();
+        assert_false(dialog.open);
+      } else {
+        assert_throws_dom('InvalidStateError',() => dialog.requestClose());
+        assert_true(dialog.open);
+      }
+    },`requestClose basic behavior ${testDescription}`);
+
+    promise_test(async (t) => {
+      const signal = await setup(t,closedby);
+      let events = [];
+      dialog.addEventListener('cancel',() => events.push('cancel'),{signal});
+      dialog.addEventListener('close',() => events.push('close'),{signal});
+      openDialog(modal);
+      assert_array_equals(events,[]);
+      if (dialog.closedBy != "none") {
+        dialog.requestClose();
+        assert_false(dialog.open);
+        assert_array_equals(events,['cancel'],'close is scheduled');
+        await new Promise(resolve => requestAnimationFrame(resolve));
+        assert_array_equals(events,['cancel','close']);
+      } else {
+        assert_throws_dom('InvalidStateError',() => dialog.requestClose());
+      }
+    },`requestClose fires both cancel and close ${testDescription}`);
 
-  promise_test(async (t) => {
-    t.add_cleanup(() => dialog.close());
-    const signal = getSignal(t);
-    let shouldPreventDefault = true;
-    dialog.addEventListener('cancel',(e) => {
-      if (shouldPreventDefault) {
-        e.preventDefault();
+    promise_test(async (t) => {
+      const signal = await setup(t,'none');
+      let events = [];
+      dialog.addEventListener('cancel',() => events.push('cancel'),{signal});
+      dialog.addEventListener('close',() => events.push('close'),{signal});
+      openDialog(modal);
+      dialog.setAttribute('closedby',closedby);
+      assert_array_equals(events,[]);
+      if (dialog.closedBy != "none") {
+        dialog.requestClose();
+        assert_false(dialog.open);
+      } else {
+        assert_throws_dom('InvalidStateError',() => dialog.requestClose());
       }
-    },{signal});
-    openDialog(modal);
-    await clickOn(dialog); // User activation
-    dialog.requestClose();
-    assert_true(dialog.open,'cancel event was cancelled - dialog shouldn\'t close');
-    shouldPreventDefault = false;
-    await clickOn(dialog); // User activation
-    dialog.requestClose();
-    assert_false(dialog.open,'cancel event was not cancelled - dialog should now close');
-  },`${modal ? "Modal:" : "Non-modal:"} requestClose can be cancelled`);
+    },`changing closedby from 'none' to '${closedby}' for ${modal ? "modal" : "modeless"} dialog`);
+
+    promise_test(async (t) => {
+      const signal = await setup(t,closedby);
+      let events = [];
+      dialog.addEventListener('cancel',() => events.push('cancel'),{signal});
+      dialog.addEventListener('close',() => events.push('close'),{signal});
+      openDialog(modal);
+      dialog.removeAttribute('closedby');
+      assert_array_equals(events,[]);
+      if (dialog.closedBy != "none") {
+        dialog.requestClose();
+        assert_false(dialog.open);
+      } else {
+        assert_throws_dom('InvalidStateError',() => dialog.requestClose());
+      }
+    },`Removing closedby when closedby='${closedby}' for ${modal ? "modal" : "modeless"} dialog`);
+
+    if (dialog.closedBy != "none") {
+      promise_test(async (t) => {
+        const signal = await setup(t,closedby);
+        let shouldPreventDefault = true;
+        dialog.addEventListener('cancel',(e) => {
+          if (shouldPreventDefault) {
+            e.preventDefault();
+          }
+        },{signal});
+        openDialog(modal);
+        await clickOn(dialog); // User activation
+        dialog.requestClose();
+        assert_true(dialog.open,'cancel event was cancelled - dialog shouldn\'t close');
+        shouldPreventDefault = false;
+        await clickOn(dialog); // User activation
+        dialog.requestClose();
+        assert_false(dialog.open,'cancel event was not cancelled - dialog should now close');
+      },`requestClose can be cancelled ${testDescription}`);
+
+      promise_test(async (t) => {
+        await setup(t,closedby);
+        openDialog(modal);
+        assert_equals(dialog.returnValue,'','Return value starts out empty');
+        const returnValue = 'The return value';
+        dialog.requestClose(returnValue);
+        assert_false(dialog.open);
+        assert_equals(dialog.returnValue,returnValue,'Return value should be set');
+        dialog.show();
+        dialog.close();
+        assert_equals(dialog.returnValue,returnValue,'Return value should not be changed by close()');
+        dialog.show();
+        dialog.close('another');
+        assert_equals(dialog.returnValue,'another','Return value changes via close(value)');
+      },`requestClose(returnValue) passes along the return value ${testDescription}`);
+
+      promise_test(async (t) => {
+        await setup(t,closedby);
+        dialog.addEventListener('cancel',(e) => e.preventDefault(),{once:true});
+        openDialog(modal);
+        dialog.returnValue = 'foo';
+        assert_equals(dialog.returnValue,'foo');
+        dialog.requestClose('This should not get saved');
+        assert_true(dialog.open,'cancelled');
+        assert_equals(dialog.returnValue,'foo','Return value should not be changed');
+      },`requestClose(returnValue) doesn't change returnvalue when cancelled ${testDescription}`);
+    }
+  });
 });
 </script>