diff --git a/testing/web-platform/tests/generic-sensor/generic-sensor-tests.js b/testing/web-platform/tests/generic-sensor/generic-sensor-tests.js
index 1779fd6bb5923..f0d37fb9f24b6 100644
--- a/testing/web-platform/tests/generic-sensor/generic-sensor-tests.js
+++ b/testing/web-platform/tests/generic-sensor/generic-sensor-tests.js
@@ -246,12 +246,15 @@ function runGenericSensorTests(sensorName,
     assert_true(verificationFunction(expected, sensor2, /*isNull=*/true));
   }, `${sensorName}: sensor reading is correct.`);
 
-  sensor_test(async t => {
+  sensor_test(async (t, sensorProvider) => {
     assert_true(sensorName in self);
     const sensor = new sensorType();
     const sensorWatcher = new EventWatcher(t, sensor, ["reading", "error"]);
     sensor.start();
 
+    const mockSensor = await sensorProvider.getCreatedSensor(sensorName);
+    await mockSensor.setSensorReading(readings);
+
     await sensorWatcher.wait_for("reading");
     const cachedTimeStamp1 = sensor.timestamp;
 
@@ -392,6 +395,7 @@ function runGenericSensorTests(sensorName,
     fastSensor.start();
 
     const mockSensor = await sensorProvider.getCreatedSensor(sensorName);
+    await mockSensor.setSensorReading(readings);
 
     const fastCounter = await new Promise((resolve, reject) => {
       let fastSensorNotifiedCounter = 0;
diff --git a/testing/web-platform/tests/resources/chromium/generic_sensor_mocks.js b/testing/web-platform/tests/resources/chromium/generic_sensor_mocks.js
index eb28c48d3a719..5e902f2523dd6 100644
--- a/testing/web-platform/tests/resources/chromium/generic_sensor_mocks.js
+++ b/testing/web-platform/tests/resources/chromium/generic_sensor_mocks.js
@@ -199,6 +199,19 @@ var GenericSensorTest = (() => {
         this.maxFrequency_ = Math.min(10, this.maxFrequency_);
       }
 
+      // Chromium applies some rounding and other privacy-related measures that
+      // can cause ALS not to report a reading when it has not changed beyond a
+      // certain threshold compared to the previous illuminance value. Make
+      // each reading return a different value that is significantly different
+      // from the previous one when setSensorReading() is not called by client
+      // code (e.g. run_generic_sensor_iframe_tests()).
+      if (type == device.mojom.SensorType.AMBIENT_LIGHT) {
+        this.activeSensors_.get(type).setSensorReading([
+          [window.performance.now() * 100],
+          [(window.performance.now() + 50) * 100]
+        ]);
+      }
+
       const initParams = new device.mojom.SensorInitParams({
         sensor: sensorPtr,
         clientReceiver: mojo.makeRequest(this.activeSensors_.get(type).client_),