HTML: review followup (#12662)

Follow up to review comments in
#12636.

Author: TimothyGu

html/webappapis/dynamic-markup-insertion/opening-the-input-stream/url.window.js

@@ -3,7 +3,7 @@ test(t => {
   t.add_cleanup(() => frame.remove());
   assert_equals(frame.contentDocument.URL, "about:blank");
   assert_equals(frame.contentWindow.location.href, "about:blank");
-  frame.contentDocument.open();
+  assert_equals(frame.contentDocument.open(), frame.contentDocument);
   assert_equals(frame.contentDocument.URL, document.URL);
   assert_equals(frame.contentWindow.location.href, document.URL);
 }, "document.open() changes document's URL (fully active document)");
@@ -26,7 +26,7 @@ async_test(t => {
 
     frame.onload = t.step_func_done(() => {
       // Now childDoc is still active but no longer fully active.
-      childDoc.open();
+      assert_equals(childDoc.open(), childDoc);
       assert_equals(childDoc.URL, blankURL);
       assert_equals(childWin.location.href, blankURL);
     });
@@ -40,6 +40,9 @@ test(t => {
   t.add_cleanup(() => frame.remove());
   const doc = frame.contentDocument;
 
+  // We do not test for win.location.href in this test due to
+  // https://github.com/whatwg/html/issues/3959.
+
   // Right now the frame is connected and it has an active document.
   assert_equals(doc.URL, "about:blank");
 
@@ -55,6 +58,9 @@ async_test(t => {
   const frame = document.createElement("iframe");
   t.add_cleanup(() => frame.remove());
 
+  // We do not test for win.location.href in this test due to
+  // https://github.com/whatwg/html/issues/3959.
+
   frame.onload = t.step_func(() => {
     const doc = frame.contentDocument;
     // Right now the frame is connected and it has an active document.