div 的 CSS

html {
  display: table;
  height: 100%;
  width: 100%;
}

body {
  display: table-cell; 
  vertical-align: middle; 

div {
  margin: auto; 
  width: 100px;
  height: 100px;
  outline: inset 100px green;
  outline-offset: -125px;
}