使用Bootstrap Modal作为注册和登录表单。出现错误时，将提供整个页面

<!-- 引入Bootstrap的CSS和JS文件 -->
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/css/bootstrap.min.css">
<script src="https://cdn.jsdelivr.net/npm/bootstrap@5.3.0/dist/js/bootstrap.bundle.min.js"></script>

<!-- 创建触发Modal的按钮 -->
<button type="button" class="btn btn-primary" data-bs-toggle="modal" data-bs-target="#myModal">
  注册/登录
</button>

<!-- Modal -->
<div class="modal fade" id="myModal" tabindex="-1" aria-labelledby="exampleModalLabel" aria-hidden="true">
  <div class="modal-dialog">
    <div class="modal-content">
      <div class="modal-header">
        <h5 class="modal-title" id="exampleModalLabel">注册/登录</h5>
        <button type="button" class="btn-close" data-bs-dismiss="modal" aria-label="Close"></button>
      </div>
      <div class="modal-body">
        <!-- 添加表单元素 -->
        <form>
          <div class="mb-3">
            <label for="username" class="form-label">用户名</label>
            <input type="text" class="form-control" id="username">
          </div>
          <div class="mb-3">
            <label for="password" class="form-label">密码</label>
            <input type="password" class="form-control" id="password">
          </div>
          <button type="submit" class="btn btn-primary">提交</button>
        </form>
      </div>
    </div>
  </div>
</div>

// 监听按钮点击事件
document.querySelector('.btn-primary').addEventListener('click', function() {
  // 显示Modal
  var myModal = new bootstrap.Modal(document.getElementById('myModal'));
  myModal.show();
});

// 后端返回错误信息
var errorMessage = '用户名或密码错误';

// 在Modal中显示错误信息
document.querySelector('.modal-body').insertAdjacentHTML('beforeend', '<div class="alert alert-danger">' + errorMessage + '</div>');