diff --git a/testing/web-platform/tests/fledge/tentative/join-leave-ad-interest-group.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/join-leave-ad-interest-group.https.sub.window.js
index e967d92618cf..3afc80eb68c3 100644
--- a/testing/web-platform/tests/fledge/tentative/join-leave-ad-interest-group.https.sub.window.js
+++ b/testing/web-platform/tests/fledge/tentative/join-leave-ad-interest-group.https.sub.window.js
@@ -2,6 +2,8 @@
 // META: script=/common/utils.js
 // META: script=resources/fledge-util.js
 
+"use strict;"
+
 // These tests are focused on joinAdInterestGroup() and leaveAdInterestGroup().
 // Most join tests do not run auctions, but instead only check the result of
 // the returned promise, since testing that interest groups are actually
diff --git a/testing/web-platform/tests/fledge/tentative/no-winner.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/no-winner.https.sub.window.js
index a944b860d0c7..8fd7df50cde0 100644
--- a/testing/web-platform/tests/fledge/tentative/no-winner.https.sub.window.js
+++ b/testing/web-platform/tests/fledge/tentative/no-winner.https.sub.window.js
@@ -3,6 +3,8 @@
 // META: script=resources/fledge-util.js
 // META: timeout=long
 
+"use strict;"
+
 // The tests in this file focus on simple auctions (one bidder, one seller, one
 // origin, one frame) which have no winning bid, either due to errors or due to
 // there being no bids, except where tests fit better with another set of tests.
diff --git a/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js
index 3df6b408a570..2284ac85b41f 100644
--- a/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js
+++ b/testing/web-platform/tests/fledge/tentative/register-ad-beacon.https.sub.window.js
@@ -3,6 +3,8 @@
 // META: script=resources/fledge-util.js
 // META: timeout=long
 
+"use strict;"
+
 promise_test(async test => {
   const uuid = generateUuid(test);
   await runReportTest(
diff --git a/testing/web-platform/tests/fledge/tentative/reporting-arguments.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/reporting-arguments.https.sub.window.js
index 69b7d2fddda4..262e0eb87e67 100644
--- a/testing/web-platform/tests/fledge/tentative/reporting-arguments.https.sub.window.js
+++ b/testing/web-platform/tests/fledge/tentative/reporting-arguments.https.sub.window.js
@@ -3,6 +3,8 @@
 // META: script=resources/fledge-util.js
 // META: timeout=long
 
+"use strict;"
+
 // Simplified version of reportTest() for validating arguments to reporting
 // methods. Only takes expressions to check in reporting methods. "uuid" is
 // optional, and one is generated if not passed one.
diff --git a/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js b/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
index 738764f65196..029df4726b09 100644
--- a/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
+++ b/testing/web-platform/tests/fledge/tentative/resources/fledge-util.js
@@ -1,3 +1,5 @@
+"use strict;"
+
 const FULL_URL = window.location.href;
 const BASE_URL = FULL_URL.substring(0, FULL_URL.lastIndexOf('/') + 1);
 const BASE_PATH = (new URL(BASE_URL)).pathname;
diff --git a/testing/web-platform/tests/fledge/tentative/send-report-to.https.sub.window.js b/testing/web-platform/tests/fledge/tentative/send-report-to.https.sub.window.js
index bb0e7873a702..de22e827a33e 100644
--- a/testing/web-platform/tests/fledge/tentative/send-report-to.https.sub.window.js
+++ b/testing/web-platform/tests/fledge/tentative/send-report-to.https.sub.window.js
@@ -3,6 +3,8 @@
 // META: script=resources/fledge-util.js
 // META: timeout=long
 
+"use strict;"
+
 promise_test(async test => {
   const uuid = generateUuid(test);
   await runReportTest(