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diff --git a/testing/web-platform/tests/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy-base-url-2.html b/testing/web-platform/tests/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy-base-url-2.html
new file mode 100644
index 0000000000..8782f3d315
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+++ b/testing/web-platform/tests/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy-base-url-2.html
@@ -0,0 +1,49 @@
+<!DOCTYPE html>
+<head>
+  <title>Deferred loading=lazy iframes load relative to the document's base URL
+         at parse-time</title>
+  <link rel="author" title="Dom Farolino" href="mailto:dom@chromium.org">
+  <link rel="author" title="Raj T" href="mailto:rajendrant@chromium.org">
+  <script src="/resources/testharness.js"></script>
+  <script src="/resources/testharnessreport.js"></script>
+  <script src="../resources/common.js"></script>
+</head>
+
+<script>
+  const below_viewport_iframe = new ElementLoadPromise("below-viewport");
+
+  let has_window_loaded = false;
+
+  async_test(t => {
+    // Change the document's base URL to a bogus one, and scroll the
+    // below-viewport iframe into view. When it loads, it should load relative
+    // to the old base URL computed at parse-time.
+    window.addEventListener("load", t.step_func(() => {
+      window.history.pushState(2, document.title,
+                               '/invalid-url-where-no-subresources-exist/')
+      has_window_loaded = true;
+      below_viewport_iframe.element().scrollIntoView();
+    }));
+
+    below_viewport_iframe.promise.then(t.step_func_done(() => {
+      assert_true(has_window_loaded,
+            "Below-viewport loading=lazy iframes do not block the " +
+            "window load event");
+      assert_true(below_viewport_iframe.element()
+                    .contentDocument.body.innerHTML.includes("<p>Subframe</p>"));
+    }));
+
+  }, "When a loading=lazy iframe is loaded, it loads relative to the " +
+     "document's base URL computed at parse-time.");
+</script>
+
+<body>
+  <div style="height:1000vh;"></div>
+  <script>
+    // Change the document's base URL so that the iframe request parses relative
+    // to it when it sets up the request at parse-time.
+    window.history.pushState(1, document.title, 'resources/')
+  </script>
+  <iframe id="below-viewport" src="subframe.html" loading="lazy" width="200px"
+          height="100px" onload="below_viewport_iframe.resolve()"</iframe>
+</body>