src/third_party/web_platform_tests/XMLHttpRequest/send-accept-language.htm

<!doctype html>
<html>
  <head>
    <title>XMLHttpRequest: send() - Accept-Language</title>
    <script src="/resources/testharness.js"></script>
    <script src="/resources/testharnessreport.js"></script>
    <link rel="help" href="https://xhr.spec.whatwg.org/#the-send()-method" data-tested-assertations="following::code[contains(text(),'Accept-Language')]/.. following::code[contains(text(),'Accept-Language')]/../following::ul[1]/li[1] following::code[contains(text(),'Accept-Language')]/../following::ul[1]/li[2]" />
  </head>
  <body>
    <div id="log"></div>
    <script>
      test(function() {
        var client = new XMLHttpRequest()
        client.open('GET', 'resources/inspect-headers.py?filter_name=accept-language', false)
        client.send(null)
        assert_regexp_match(client.responseText, /accept-language:\s.+/)
      }, 'Send "sensible" default value, whatever that means')
      test(function() {
        var client = new XMLHttpRequest()
        client.open("GET", "resources/inspect-headers.py?filter_name=accept-language", false)
        client.setRequestHeader("Accept-Language", "x-GameSpeak")
        client.send(null)
        assert_equals(client.responseText, "accept-language: x-GameSpeak\n")
      })
    </script>
  </body>
</html>