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diff --git a/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/remove-initial-about-blankness.window.js b/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/remove-initial-about-blankness.window.js
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+++ b/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/remove-initial-about-blankness.window.js
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+// This tests the issues discussed in https://github.com/whatwg/html/issues/4299
+// and fixed in https://github.com/whatwg/html/pull/6567.
+
+// Note: because browsers do not interoperate on the spec's notion of window reuse (see e.g. https://crbug.com/778318)
+// we pick a specific interoperable test case, which is "currently on initial about:blank, but loading something".
+
+async_test(t => {
+  const iframe = document.createElement("iframe");
+
+  // We can't just leave it at the actual initial about:blank because of the interop issues mentioned above.
+  // So put it in the "currently on initial about:blank, but loading something" state which interoperably does Window
+  // reuse.
+  iframe.src = "/common/blank.html";
+
+  // Create the Window object. It will be for the initial about:blank since the load of /common/blank.html hasn't
+  // completed.
+  document.body.append(iframe);
+
+  // Store a string on that Window object so we can later test if it's reused.
+  iframe.contentWindow.persistedString = "Hello world!";
+
+  // This will reset the initial about:blank-ness. But, it will also cancel any ongoing loads.
+  iframe.contentDocument.open();
+
+  // So, re-start the load of /common/blank.html.
+  iframe.src = "/common/blank.html";
+
+  // When the load finally happens, will it reuse the Window object or not?
+  // Because document.open() resets the initial about:blank-ness, it will *not* reuse the Window object.
+  // The point of the test is to assert that.
+  iframe.addEventListener("load", t.step_func_done(() => {
+    assert_equals(
+      iframe.contentDocument.URL,
+      iframe.src,
+      "Prerequisite check: we are getting the right load event"
+    );
+
+    assert_equals(iframe.contentWindow.persistedString, undefined);
+  }), { once: true });
+}, "document.open() removes the initial about:blank-ness of the document");
+
+// This test is redundant with others in WPT but it's intended to make it clear that document.open() is the
+// distinguishing factor. It does the same exact thing but without document.open() and with the resulting final assert
+// flipped.
+async_test(t => {
+  const iframe = document.createElement("iframe");
+  iframe.src = "/common/blank.html";
+  document.body.append(iframe);
+
+  iframe.contentWindow.persistedString = "Hello world!";
+
+  // NO document.open() call.
+
+  iframe.src = "/common/blank.html";
+
+  iframe.addEventListener("load", t.step_func_done(() => {
+    assert_equals(
+      iframe.contentDocument.URL,
+      iframe.src,
+      "Prerequisite check: we are getting the right load event"
+    );
+
+    assert_equals(iframe.contentWindow.persistedString, "Hello world!");
+  }), { once: true });
+}, "Double-check: without document.open(), Window reuse indeed happens");