FormData should not contain an entry for the submitter (#27439)

FormData should not contain an entry for the submit button that was used to submit the form.

html/semantics/forms/the-form-element/form-requestsubmit.html

@@ -173,4 +173,24 @@
   }));
 }, 'The value of the submitter should be appended, and form* ' +
     'attributes of the submitter should be handled.');
+
+test(() => {
+  document.body.insertAdjacentHTML('afterbegin', `<form>
+      <input name="n1" value="v1">
+      <button type="submit" name="n2" value="v2"></button>
+      </form>
+      <form id="form2"></form>`);
+  let form = document.querySelector('form');
+  let formDataInEvent = null;
+  let submitter = form.querySelector('button[type=submit]');
+  form.addEventListener('submit', e => {
+    e.preventDefault();
+    formDataInEvent = new FormData(e.target);
+  });
+
+  form.requestSubmit(submitter);
+  assert_equals(formDataInEvent.get('n1'), 'v1');
+  assert_false(formDataInEvent.has('n2'));
+}, 'The constructed FormData object should not contain an entry for the submit button that was used to submit the form.');
+
 </script>