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diff --git a/testing/web-platform/tests/webaudio/the-audio-api/the-channelmergernode-interface/active-processing.https.html b/testing/web-platform/tests/webaudio/the-audio-api/the-channelmergernode-interface/active-processing.https.html
new file mode 100644
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+++ b/testing/web-platform/tests/webaudio/the-audio-api/the-channelmergernode-interface/active-processing.https.html
@@ -0,0 +1,93 @@
+<!doctype html>
+<html>
+  <head>
+    <title>
+      Test Active Processing for ChannelMergerNode
+    </title>
+    <script src="/resources/testharness.js"></script>
+    <script src="/resources/testharnessreport.js"></script>
+    <script src="/webaudio/resources/audit.js"></script>
+  </head>
+
+  <body>
+    <script id="layout-test-code">
+      // AudioProcessor that sends a message to its AudioWorkletNode whenver the
+      // number of channels on its input changes.
+      let filePath =
+          '../the-audioworklet-interface/processors/active-processing.js';
+
+      const audit = Audit.createTaskRunner();
+
+      let context;
+
+      audit.define('initialize', (task, should) => {
+        // Create context and load the module
+        context = new AudioContext();
+        should(
+            context.audioWorklet.addModule(filePath),
+            'AudioWorklet module loading')
+            .beResolved()
+            .then(() => task.done());
+      });
+
+      audit.define('test', (task, should) => {
+        const src = new OscillatorNode(context);
+
+        // Number of inputs for the ChannelMergerNode.  Pretty arbitrary, but
+        // should not be 1.
+        const numberOfInputs = 7;
+        const merger =
+            new ChannelMergerNode(context, {numberOfInputs: numberOfInputs});
+
+        const testerNode =
+            new AudioWorkletNode(context, 'active-processing-tester', {
+              // Use as short a duration as possible to keep the test from
+              // taking too much time.
+              processorOptions: {testDuration: .5},
+            });
+
+        // Expected number of output channels from the merger node.  We should
+        // start with the number of inputs, because the source (oscillator) is
+        // actively processing.  When the source stops, the number of channels
+        // should change to 1.
+        const expectedValues = [numberOfInputs, 1];
+        let index = 0;
+
+        testerNode.port.onmessage = event => {
+          let count = event.data.channelCount;
+          let finished = event.data.finished;
+
+          // If we're finished, end testing.
+          if (finished) {
+            // Verify that we got the expected number of changes.
+            should(index, 'Number of distinct values')
+                .beEqualTo(expectedValues.length);
+
+            task.done();
+            return;
+          }
+
+          if (index < expectedValues.length) {
+            // Verify that the number of channels matches the expected number of
+            // channels.
+            should(count, `Test ${index}: Number of convolver output channels`)
+                .beEqualTo(expectedValues[index]);
+          }
+
+          ++index;
+        };
+
+        // Create the graph and go
+        src.connect(merger).connect(testerNode).connect(context.destination);
+        src.start();
+
+        // Stop the source after a short time so we can test that the channel
+        // merger changes to not actively processing and thus produces a single
+        // channel of silence.
+        src.stop(context.currentTime + .1);
+      });
+
+      audit.run();
+    </script>
+  </body>
+</html>