LayoutTests/fast/forms/submit-button-submit-event.html

<!DOCTYPE html>
<html>
<body>
<script src="../../resources/js-test.js"></script>
<form id="testForm" action="" method="GET" style="display:none">
<input type="text" name="title" value="foo">
<button type="submit" name="action" value="submit" id="testButton">Submit</button>
</form>
<script>
description("Tests that SubmitEvent.submitter property is properly set for <button type='submit'>.");
jsTestIsAsync = true;

onload = () => {
    testButton =  document.getElementById("testButton");
    document.getElementById("testForm").onsubmit = (_submitEvent) => {
        submitEvent = _submitEvent;
        shouldBe("submitEvent.submitter", "testButton");
        finishJSTest();
    };
    testButton.click();
};
</script>
</body>
</html>