serial is a wrapper around the window.navigator.serial. This package does not provide any additional API, but merely helps to make the dart:html package work "out of the box" without the need of manually writing any javascript code.
In order to access serial ports on web, you need your web page to open from an HTTPS url.
Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.
More information: https://developer.mozilla.org/en-US/docs/Web/API/SerialPort
import 'dart:html';
import 'package:serial/serial.dart';
final port = await window.navigator.serial.requestPort();
await port.open(baudRate: 9600);
final writer = port.writable.writer;
await writer.ready;
await writer.write(Uint8List.fromList('Hello World.'.codeUnits));
await writer.ready;
await writer.close();