diff --git a/testing/web-platform/meta/MANIFEST.json b/testing/web-platform/meta/MANIFEST.json
index f5f9403ec246c..c18887e529378 100644
--- a/testing/web-platform/meta/MANIFEST.json
+++ b/testing/web-platform/meta/MANIFEST.json
@@ -368902,6 +368902,12 @@
      {}
     ]
    ],
+   "html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js": [
+    [
+     "/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.html",
+     {}
+    ]
+   ],
    "html/webappapis/dynamic-markup-insertion/opening-the-input-stream/no-new-global.window.js": [
     [
      "/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/no-new-global.window.html",
@@ -613663,6 +613669,10 @@
    "4efbb863c6372a3ee04d11f38d7ee56a44a2ac7d",
    "testharness"
   ],
+  "html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js": [
+   "34e73146a9d8e4ecab1e74a0079cd721e7306e59",
+   "testharness"
+  ],
   "html/webappapis/dynamic-markup-insertion/opening-the-input-stream/no-new-global.window.js": [
    "d4a9296fca66d3017c267a8d8951bef2dcbee238",
    "testharness"
diff --git a/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js b/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js
new file mode 100644
index 0000000000000..34e73146a9d8e
--- /dev/null
+++ b/testing/web-platform/tests/html/webappapis/dynamic-markup-insertion/opening-the-input-stream/mutation-observer.window.js
@@ -0,0 +1,19 @@
+async_test(t => {
+  const frame = document.body.appendChild(document.createElement("iframe"));
+  t.add_cleanup(() => { frame.remove(); });
+  const originalHTMLElement = frame.contentDocument.documentElement;
+  assert_equals(originalHTMLElement.localName, "html");
+  const observer = new frame.contentWindow.MutationObserver(t.step_func_done(records => {
+    // Even though we passed `subtree: true` to observer.observe, due to the
+    // fact that "replace all" algorithm removes children with the "suppress
+    // observers flag" set, we still only get the html element as the sole
+    // removed node.
+    assert_equals(records.length, 1);
+    assert_equals(records[0].type, "childList");
+    assert_equals(records[0].target, frame.contentDocument);
+    assert_array_equals(records[0].addedNodes, []);
+    assert_array_equals(records[0].removedNodes, [originalHTMLElement]);
+  }));
+  observer.observe(frame.contentDocument, { childList: true, subtree: true });
+  assert_equals(frame.contentDocument.open(), frame.contentDocument);
+}, "document.open() should inform mutation observer of node removal");