third_party/web_platform_tests/fetch/api/response/response-trailer.html

<!doctype html>
<title>trailer() test</title>
<script src=/resources/testharness.js></script>
<script src=/resources/testharnessreport.js></script>
<div id=log></div>
<!-- based on /XMLHttpRequest/getresponseheader-chunked-trailer.htm -->
<script>
promise_test(() => {
  return fetch("/XMLHttpRequest/resources/chunked.py").then(res => {
    assert_equals(res.headers.get("Trailer"), "X-Test-Me")
    assert_equals(res.headers.get("X-Test-Me"), null)
    assert_equals(res.headers.get("Content-Type"), "text/plain")
    return Promise.all([
      res.text().then(text => {
        assert_equals(text, "First chunk\r\nSecond chunk\r\nYet another (third) chunk\r\nYet another (fourth) chunk\r\n")
      }),
      res.trailer.then(trailers => {
        assert_equals(trailers.get("X-Test-Me"), "Trailer header value")
        assert_throws(new TypeError, () => trailers.append("Immutable-My-Ass", "Hi!"))
      })
    ])
  })
})
</script>