如何表明必填字段？

<form>
 <label for="name">姓名：</label>
 <input type="text" id="name" name="name" required>
  <br>
 <label for="email">邮箱：</label>
 <input type="email" id="email" name="email" required>
  <br>
 <label for="message">留言：</label>
 <textarea id="message" name="message" required></textarea>
  <br>
 <input type="submit" value="提交">
</form>

const express = require('express');
const bodyParser = require('body-parser');
const Joi = require('joi');

const app = express();
app.use(bodyParser.json());

app.post('/submit', (req, res) => {
  const schema = Joi.object({
    name: Joi.string().required(),
    email: Joi.string().email().required(),
    message: Joi.string().required(),
  });

  const validationResult = schema.validate(req.body);
  if (validationResult.error) {
    return res.status(400).send(validationResult.error.details[0].message);
  }

  // 处理表单数据
  // ...

  res.send('表单提交成功！');
});

app.listen(3000, () => {
  console.log('服务器已启动，监听端口3000');
});