diff --git a/resource-timing/TAO-crossorigin-port.sub.html b/resource-timing/TAO-crossorigin-port.sub.html
deleted file mode 100644
index 0601217d0ae3a7..00000000000000
--- a/resource-timing/TAO-crossorigin-port.sub.html
+++ /dev/null
@@ -1,25 +0,0 @@
-<!DOCTYPE html>
-<html>
-<head>
-<meta charset="utf-8" />
-<title>Resource Timing TAO - "null" and opaque origin</title>
-<link rel="author" title="Google" href="http://www.google.com/" />
-<link rel="help" href="https://www.w3.org/TR/resource-timing-2/#timing-allow-origin"/>
-<script src="/resources/testharness.js"></script>
-<script src="/resources/testharnessreport.js"></script>
-<script>
-
-const t = async_test("Makes sure that the iframe passed the test and had an entry which passed the timing allow check");
-window.addEventListener("message", t.step_func_done(e=>{
-    assert_equals(e.data, "PASS");
-}));
-</script>
-</head>
-<body>
-<h1>Description</h1>
-<p>This test validates that for a cross origin resource with different ports, the timing allow check algorithm will fail when the value of Timing-Allow-Origin value has the right host but the wrong port in it.</p>
-<div id="log"></div>
-<!-- The frame is being requested on the default port ([0]), while the subresource in it will be requested on a separate port ([1]) -->
-<iframe id="frameContext" src="{{location[scheme]}}://{{host}}:{{ports[http][0]}}/resource-timing/resources/iframe-TAO-crossorigin-port.sub.html"></iframe>
-</body>
-</html>
diff --git a/resource-timing/TAO-port-mismatch-means-crossorigin.html b/resource-timing/TAO-port-mismatch-means-crossorigin.html
new file mode 100644
index 00000000000000..1dc27fcc373039
--- /dev/null
+++ b/resource-timing/TAO-port-mismatch-means-crossorigin.html
@@ -0,0 +1,35 @@
+<!DOCTYPE html>
+<html>
+<head>
+<meta charset="utf-8" />
+<title>TAO - port mismatch must fail the check</title>
+<link rel="author" title="Google" href="http://www.google.com/" />
+<link rel="help" href="https://www.w3.org/TR/resource-timing-2/#sec-timing-allow-origin"/>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script src="/common/get-host-info.sub.js"></script>
+<script src="resources/entry-invariants.js"></script>
+<script>
+
+const {ORIGINAL_HOST, HTTP_PORT2} = get_host_info();
+
+// The main page is being requested on the default port (HTTP_PORT), while the
+// subresource will be requested on a separate port (HTTP_PORT2). The response
+// will have a Timing-Allow-Origin header value with the second port so this
+// page's origin should not be a match.
+const url = `${location.protocol}//${ORIGINAL_HOST}:${HTTP_PORT2}` +
+            `/resource-timing/resources/TAOResponse.py?` +
+            `tao=origin_port_${HTTP_PORT2}`;
+attribute_test(
+  fetch, url, invariants.assert_tao_failure_resource,
+  "Make sure that the port matters for the TAO check");
+
+</script>
+</head>
+<body>
+<h1>Description</h1>
+<p>This test validates that for a cross origin resource with different ports,
+the timing allow check algorithm will fail when the value of
+Timing-Allow-Origin value has the right host but the wrong port in it.</p>
+</body>
+</html>
diff --git a/resource-timing/resources/entry-invariants.js b/resource-timing/resources/entry-invariants.js
index eb885e6de20d04..fa852555724f57 100644
--- a/resource-timing/resources/entry-invariants.js
+++ b/resource-timing/resources/entry-invariants.js
@@ -164,7 +164,32 @@ const invariants = {
 
     assert_equals(entry.fetchStart, entry.startTime,
       "fetchStart must equal startTime");
+  },
+
+  assert_tao_failure_resource: entry => {
+    assert_equals(entry.entryType, "resource", "entryType must always be 'resource'");
+
+    assert_positive_(entry, [
+      "startTime",
+      "duration",
+    ]);
+
+    assert_zeroed_(entry, [
+      "redirectStart",
+      "redirectEnd",
+      "domainLookupStart",
+      "domainLookupEnd",
+      "connectStart",
+      "connectEnd",
+      "secureConnectionStart",
+      "requestStart",
+      "responseStart",
+      "transferSize",
+      "encodedBodySize",
+      "decodedBodySize",
+    ]);
   }
+
 };
 
 // Given a resource-loader, a path (a relative path or absolute URL), and a
