完全居中

  <div class="parent">
    <p class="children">完全居中</p>
  </div>

• 知道宽高

（1）绝对定位

.parent {
    width: 300px;
    height: 400px;
    margin: 0 auto;
    border: 1px solid red;
    position: relative;
  }
  .children {
    position: absolute;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    margin: auto;
    width: 100px;
    height: 50px;
  }

（2）负margin

  .parent {
    width: 300px;
    height: 400px;
    margin: 0 auto;
    border: 1px solid red;
    position: relative;
  }
  .children {
    position: absolute;
    top: 50%;
    left: 50%;
    margin: -50px 0 0 -25px;
    width: 100px;
    height: 50px;
  }

• 不知道宽高

（1）transform

  .parent {
    width: 300px;
    height: 400px;
    margin: 0 auto;
    border: 1px solid red;
    position: relative;
  }
  .children {
    position: absolute;
    top: 50%;
    left: 50%;
    margin: auto;
    transform: translate(-50%, -50%);
  }

（2）table-cell + vertical-align: center

      .parent {        width: 100px;        margin: 0 auto;        border: 1px solid red;        display: table-cell;        vertical-align: center;        text-align: center;      }      .children {        margin: 0 auto;      }

（3）弹性布局 flex

      .parent {        display: flex;        align-items: center;        justify-content: center;      }

（4）display：inline-block

.parent {  width: 300px;  height: 500px;  border: 1px solid red;  text-align: center;  overflow: auto;}.parent:after, .children {  display: inline-block;  vertical-align: middle;}.parent:after {  content: '';  height: 100%;}