diff --git a/testing/web-platform/tests/eventsource/format-field-id-3.window.js b/testing/web-platform/tests/eventsource/format-field-id-3.window.js
new file mode 100644
index 000000000000..3766fbf7bb1e
--- /dev/null
+++ b/testing/web-platform/tests/eventsource/format-field-id-3.window.js
@@ -0,0 +1,56 @@
+const ID_PERSISTS = 1,
+ID_RESETS_1 = 2,
+ID_RESETS_2 = 3;
+
+async_test(testPersist, "EventSource: lastEventId persists");
+async_test(testReset(ID_RESETS_1), "EventSource: lastEventId resets");
+async_test(testReset(ID_RESETS_2), "EventSource: lastEventId resets (id without colon)");
+
+function testPersist(t) {
+    const source = new EventSource("resources/last-event-id2.py?type=" + ID_PERSISTS);
+    let counter = 0;
+    t.add_cleanup(() => source.close());
+    source.onmessage = t.step_func(e => {
+      counter++;
+      if (counter === 1) {
+          assert_equals(e.lastEventId, "1");
+          assert_equals(e.data, "1");
+      } else if (counter === 2) {
+          assert_equals(e.lastEventId, "1");
+          assert_equals(e.data, "2");
+      } else if (counter === 3) {
+          assert_equals(e.lastEventId, "2");
+          assert_equals(e.data, "3");
+      } else if (counter === 4) {
+          assert_equals(e.lastEventId, "2");
+          assert_equals(e.data, "4");
+          t.done();
+      } else {
+          assert_unreached();
+      }
+  });
+}
+
+function testReset(type) {
+  return function (t) {
+    const source = new EventSource("resources/last-event-id2.py?type=" + type);
+    let counter = 0;
+    t.add_cleanup(() => source.close());
+    source.onmessage = t.step_func(e => {
+      counter++;
+      if (counter === 1) {
+        assert_equals(e.lastEventId, "1");
+        assert_equals(e.data, "1");
+      } else if (counter === 2) {
+        assert_equals(e.lastEventId, "");
+        assert_equals(e.data, "2");
+      } else if (counter === 3) {
+        assert_equals(e.lastEventId, "");
+        assert_equals(e.data, "3");
+        t.done();
+      } else {
+          assert_unreached();
+      }
+    });
+  }
+}
diff --git a/testing/web-platform/tests/eventsource/resources/last-event-id2.py b/testing/web-platform/tests/eventsource/resources/last-event-id2.py
new file mode 100644
index 000000000000..4f133d707d1f
--- /dev/null
+++ b/testing/web-platform/tests/eventsource/resources/last-event-id2.py
@@ -0,0 +1,23 @@
+ID_PERSISTS = 1
+ID_RESETS_1 = 2
+ID_RESETS_2 = 3
+
+def main(request, response):
+  response.headers.set(b"Content-Type", b"text/event-stream")
+  try:
+    test_type = int(request.GET.first(b"type", ID_PERSISTS))
+  except:
+    test_type = ID_PERSISTS
+
+  if test_type == ID_PERSISTS:
+    return b"id: 1\ndata: 1\n\ndata: 2\n\nid: 2\ndata:3\n\ndata:4\n\n"
+
+  elif test_type == ID_RESETS_1:
+    return b"id: 1\ndata: 1\n\nid:\ndata:2\n\ndata:3\n\n"
+
+  # empty id field without colon character (:) should also reset
+  elif test_type == ID_RESETS_2:
+    return b"id: 1\ndata: 1\n\nid\ndata:2\n\ndata:3\n\n"
+
+  else:
+    return b"data: invalid_test\n\n"