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chromium/third_party/blink/web_tests/plugins/embed-src-change-usecounter.html 

<!DOCTYPE html>
<script src="../resources/testharness.js"></script>
<script src="../resources/testharnessreport.js"></script>

<embed id=embedid src="../http/tests/resources/test.mp4">

<script>
test(() => {
  embedid.src = '../http/tests/resources/test.webm';
  assert_true(internals.isUseCounted(document, 3749));
}, 'Verifies that changing embed src gets use counted.');
</script>