diff --git a/common/square.png b/common/square.png
new file mode 100644
index 00000000000000..01c9666a8de9d5
Binary files /dev/null and b/common/square.png differ
diff --git a/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy.tentative.html b/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy.tentative.html
index 0a882ef25bc19a..1a5f8f7407e340 100644
--- a/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy.tentative.html
+++ b/html/semantics/embedded-content/the-iframe-element/iframe-loading-lazy.tentative.html
@@ -2,38 +2,56 @@
 <head>
   <title>Iframes with loading='lazy' load when in the viewport</title>
   <link rel="author" title="Scott Little" href="mailto:sclittle@chromium.org">
-  <link rel="help" href="https://github.com/scott-little/lazyload">
+  <link rel="author" title="Dom Farolino" href="mailto:dom@chromium.org">
+  <link rel="help" href="https://github.com/whatwg/html/pull/5579">
   <script src="/resources/testharness.js"></script>
   <script src="/resources/testharnessreport.js"></script>
 </head>
 
 <script>
-  const t = async_test("Test that iframes with loading='lazy' load once they " +
-                       "enter the viewport.");
+  const t_in_viewport =
+    async_test('In-viewport iframes load eagerly');
+  const t_in_viewport_srcdoc=
+    async_test('In-viewport srcdoc iframes load eagerly');
+  const t_below_viewport =
+    async_test('Below-viewport iframes load lazily');
+  const t_below_viewport_srcdoc =
+    async_test('Below-viewport srcdoc iframes load lazily');
 
   let has_window_loaded = false;
-  let has_in_viewport_loaded = false;
 
-  const in_viewport_iframe_onload = t.step_func(() => {
-    assert_false(has_in_viewport_loaded,
-                 "The in_viewport element should load only once.");
-    has_in_viewport_loaded = true;
+  const in_viewport_iframe_onload = t_in_viewport.step_func_done(() => {
+    assert_false(has_window_loaded,
+      "The in_viewport iframe should not block the load event");
   });
 
-  window.addEventListener("load", t.step_func(() => {
-    assert_true(has_in_viewport_loaded,
-                "The in_viewport element should have loaded before " +
-                "window.load().");
+  const in_viewport_srcdoc_iframe_onload = t_in_viewport_srcdoc.step_func_done(() => {
     assert_false(has_window_loaded,
-                 "The window.load() event should only fire once.");
+      "The in_viewport srcdoc iframe should not block the load event");
+  });
+
+  window.addEventListener("load", () => {
     has_window_loaded = true;
-    document.getElementById("below_viewport").scrollIntoView();
-  }));
+    document.getElementById("below_viewport_srcdoc").scrollIntoView();
+  });
+
+  const below_viewport_iframe_onload = t_below_viewport.step_func_done(() => {
+    assert_true(has_window_loaded,
+                "The window.load() event should have fired before " +
+                "the below-viewport iframe loads");
+  });
 
-  const below_viewport_iframe_onload = t.step_func_done(() => {
+  // Must make this accessible to the srcdoc iframe's body.
+  window.below_viewport_srcdoc_iframe_subresource_onload = t_below_viewport_srcdoc.step_func(() => {
     assert_true(has_window_loaded,
                 "The window.load() event should have fired before " +
-                "below_viewport loaded.");
+                "the below-viewport srcdoc iframe's subresource loads");
+  });
+
+  const below_viewport_srcdoc_iframe_onload = t_below_viewport_srcdoc.step_func_done(() => {
+    assert_true(has_window_loaded,
+                "The window.load() event should have fired before " +
+                "the below-viewport srcdoc iframe loads");
   });
 </script>
 
@@ -41,13 +59,24 @@
   <iframe id="in_viewport" src="resources/subframe.html?in-viewport"
           loading="lazy" width="200px" height="100px"
           onload="in_viewport_iframe_onload();"></iframe>
-  <div style="height:1000vh;"></div>
+  <iframe id="in_viewport_srcdoc"
+          srcdoc="<body><img src='/common/square.png?in-viewport'></body>"
+          loading="lazy" width="200px" height="100px"
+          onload="in_viewport_srcdoc_iframe_onload();"></iframe>
+
+  <div style="height:2000vh;"></div>
   <iframe id="below_viewport" src="resources/subframe.html?below-viewport"
           loading="lazy" width="200px" height="100px"
           onload="below_viewport_iframe_onload();"></iframe>
+  <iframe id="below_viewport_srcdoc"
+          srcdoc="<body><img src='/common/square.png?below-viewport'
+                  onload='parent.below_viewport_srcdoc_iframe_subresource_onload();'></body>"
+          loading="lazy" width="200px" height="100px"
+          onload="below_viewport_srcdoc_iframe_onload();"></iframe>
+
 
   <!-- This async script loads very slowly in order to ensure that, if the
-       below_viewport element has started loading, it has a chance to finish
+       below_viewport* elements have started loading, it has a chance to finish
        loading before window.load() happens, so that the test will dependably
        fail in that case instead of potentially passing depending on how long
        different resource fetches take. -->
