定宽高的水平垂直居中

绝对定位和负magin值

效果预览

<div class="parent">
  <div class="child">div>
div>  

div.parent {
  width: 400px;
  height: 400px;
  background: #EDEDED;
  position: relative;
}
div.child {
  width: 200px;
  height: 200px;
  background: green;
  position: absolute;
  left: 50%;
  top: 50%;
  margin-left: -100px;
  margin-top: -100px;
}

绝对定位 + left/right/bottom/top + margin

div.parent {
  width: 400px;
  height: 400px;
  background: #EDEDED;
  position: relative;
}
div.child {
  width: 200px;
  height: 200px;
  background: green;
  position: absolute;
  left: 0;
  top: 0;
  bottom: 0;
  right: 0;
  margin: auto;
}

flex

这种方法在不定宽高的时候也可以使用，页面结构如上，效果预览

div.parent {
  width: 400px;
  height: 400px;
  background: #EDEDED;
  display: flex;
  justify-content:center;
  align-items: center;              

}
div.child {
  width: 300px;
  height: 200px;
  background: green;
}

绝对定位+transform

这个方法在不定宽高中也使用过，页面结构不变，效果预览

div.parent {
  width: 400px;
  height: 400px;
  background: #EDEDED;
  position: relative;       
}
div.child {
  width: 100px;
  height: 100px;
  position: absolute;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
  background: green;
}

table-cell + vertical-align + inline-block

先提醒一下，vertical-align的值是middle，而不是center

页面结构不变，效果预览

div.parent {
  width: 400px;
  height: 400px;
  background: #EDEDED;
  display: table-cell;
  text-align: center;
  vertical-align: middle;
}
div.child {
  width: 100px;
  height: 100px;
  display: inline-block;
  background: green;
}