水平垂直居中

使用 grid，它是做二维布局的，但是只有一个子元素时，一维布局与二维布局就一样了。结合 justify-content/justify-items 和 align-content/align-items 就有四种方案

.container {
  display: grid;
  justify-content: center;
  align-content: center;
}
.container {
  display: grid;
  justify-content: center;
  align-items: center;
}
.container {
  display: grid;
  justify-items: center;
  align-content: center;
}
.container {
  display: grid;
  justify-items: center;
  align-items: center;
}