chromium/src/third_party/blink/web_tests/http/tests/xmlhttprequest/resources/multiple-send.html

<script>
    var xhr = new XMLHttpRequest();
    xhr.open("GET", "./get.txt", true);
    xhr.onreadystatechange = function () {
        if (xhr.readyState != 4)
            return;
        xhr.open("GET", "./get.txt", true);
        xhr.send(null);
    };
    xhr.send(null);
</script>