css背景图片全屏显示，缩放始终水平垂直居中

以下是两种CSS实现背景图片全屏显示并始终水平垂直居中的方法：

1. 使用background-size和background-position属性：

body {
  background-image: url(images/background-photo.jpg);
  background-repeat: no-repeat;
  background-attachment: fixed;
  background-size: cover;
  background-position: center center;
}

2. 使用background-size和flexbox布局：

body {
  background-image: url(images/background-photo.jpg);
  background-repeat: no-repeat;
  background-attachment: fixed;
  background-size: cover;
  display: flex;
  justify-content: center;
  align-items: center;
}