diff --git a/testing/web-platform/tests/common/performance-timeline-utils.js b/testing/web-platform/tests/common/performance-timeline-utils.js
index 6845d6cbc689..3efce5b5d8b3 100644
--- a/testing/web-platform/tests/common/performance-timeline-utils.js
+++ b/testing/web-platform/tests/common/performance-timeline-utils.js
@@ -42,3 +42,10 @@ function test_entries(actualEntries, expectedEntries) {
     }
   })
 }
+
+function delayedLoadListener(callback) {
+  window.addEventListener('load', function() {
+    // TODO(cvazac) Remove this setTimeout when spec enforces sync entries.
+    step_timeout(callback, 0)
+  })
+}
diff --git a/testing/web-platform/tests/server-timing/cross_origin.https.html b/testing/web-platform/tests/server-timing/cross_origin.https.html
index 1421b38a05eb..d31c7b006caf 100644
--- a/testing/web-platform/tests/server-timing/cross_origin.https.html
+++ b/testing/web-platform/tests/server-timing/cross_origin.https.html
@@ -2,7 +2,7 @@
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
     <script src="/common/performance-timeline-utils.js"></script>
     <script src="/common/get-host-info.sub.js"></script>
     <script>
@@ -20,7 +20,7 @@
         document.getElementsByTagName('script')[0].parentNode.appendChild(img)
       })
 
-      window.addEventListener('load', function() {
+      delayedLoadListener(function() {
         function assertServerTimingEntries(url, expectedEntryCount) {
           test_equals(performance.getEntriesByName(url)[0].serverTiming.length,
             expectedEntryCount,
diff --git a/testing/web-platform/tests/server-timing/navigation_timing_idl.https.html b/testing/web-platform/tests/server-timing/navigation_timing_idl.https.html
index 290bb889b149..d911d64e47ce 100644
--- a/testing/web-platform/tests/server-timing/navigation_timing_idl.https.html
+++ b/testing/web-platform/tests/server-timing/navigation_timing_idl.https.html
@@ -2,10 +2,11 @@
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="/common/performance-timeline-utils.js"></script>
     <script>
       setup({explicit_done: true})
-      window.addEventListener('load', function(){
+      delayedLoadListener(function(){
         assert_not_equals(typeof performance.getEntriesByType('navigation')[0].serverTiming, 'undefined',
           'An instance of `PerformanceNavigationTiming` should have a `serverTiming` attribute.')
         done()
diff --git a/testing/web-platform/tests/server-timing/resource_timing_idl.https.html b/testing/web-platform/tests/server-timing/resource_timing_idl.https.html
index d2c3c9274eab..b1d8699cc605 100644
--- a/testing/web-platform/tests/server-timing/resource_timing_idl.https.html
+++ b/testing/web-platform/tests/server-timing/resource_timing_idl.https.html
@@ -2,10 +2,11 @@
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="/common/performance-timeline-utils.js"></script>
     <script>
       setup({explicit_done: true})
-      window.addEventListener('load', function(){
+      delayedLoadListener(function(){
         assert_not_equals(typeof performance.getEntriesByType('resource')[0].serverTiming, 'undefined',
           'An instance of `PerformanceResourceTiming` should have a `serverTiming` attribute.')
         done()
diff --git a/testing/web-platform/tests/server-timing/server_timing_header-parsing.https.html b/testing/web-platform/tests/server-timing/server_timing_header-parsing.https.html
index 156dfcd344f3..737cc08045ae 100644
--- a/testing/web-platform/tests/server-timing/server_timing_header-parsing.https.html
+++ b/testing/web-platform/tests/server-timing/server_timing_header-parsing.https.html
@@ -7,7 +7,7 @@ tests generated by:
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
     <script src="/common/performance-timeline-utils.js"></script>
     <script>
       setup({explicit_done: true})
@@ -49,6 +49,6 @@ tests generated by:
         document.getElementsByTagName('head')[0].appendChild(script)
         urlToIndex[script.src] = i
       }
-      window.addEventListener('load', runTests)
+      delayedLoadListener(runTests)
     </script>
 </head>
diff --git a/testing/web-platform/tests/server-timing/service_worker_idl.https.html b/testing/web-platform/tests/server-timing/service_worker_idl.https.html
index 1c1be8995b33..5c493fdcb0ab 100644
--- a/testing/web-platform/tests/server-timing/service_worker_idl.https.html
+++ b/testing/web-platform/tests/server-timing/service_worker_idl.https.html
@@ -2,7 +2,7 @@
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
     <script>
       (async () => {
         const scope = 'does/not/exist'
diff --git a/testing/web-platform/tests/server-timing/test_server_timing.https.html b/testing/web-platform/tests/server-timing/test_server_timing.https.html
index 2d43aa2d43b7..4f3554aa4a45 100644
--- a/testing/web-platform/tests/server-timing/test_server_timing.https.html
+++ b/testing/web-platform/tests/server-timing/test_server_timing.https.html
@@ -2,12 +2,12 @@
 <head>
     <meta charset='utf-8' />
     <script src="/resources/testharness.js"></script>
-    <script src='/resources/testharnessreport.js'></script>
+    <script src="/resources/testharnessreport.js"></script>
     <script src="/common/performance-timeline-utils.js"></script>
     <script>
       setup({explicit_done: true})
 
-      window.addEventListener('load', function() {
+      delayedLoadListener(function() {
         // there should be exactly three server-timing entries, 2 for document, 1 for img#one
         test_entries(performance.getEntriesByType('navigation')[0].serverTiming, [{
           duration: 1.1,
