话不多说直接上代码

html代码统一如下：

<div class="wrap">
	<div class="inner">
	    innerBox
	</div>
</div>

 css代码统一如下：

定位方法A position + relative + absolute

    .wrap {
      width: 300px;
      height: 300px;
      background: yellow;
      /*加颜色是为了看界面效果*/
      position: relative;
    }

    .inner {
      width: 40px;
      height: 40px;
      position: absolute;
      left: 0;
      right: 0;
      top: 0;
      bottom: 0;
      margin: auto;
      background: orange;
      /*加颜色是为了看界面效果*/
    }

 定位方法B position + relative + absolute + margin

    .wrap {
      width: 300px;
      height: 300px;
      background: yellow;
      /*加颜色是为了看界面效果*/
      position: relative;
    }

    .inner {
      width: 40px;
      height: 40px;
      position: absolute;
      top: 50%;
      left: 50%;
      margin-left: -20px;
      margin-top: -20px;
      background: orange;
      /*加颜色是为了看界面效果*/
    }

 定位方法C position + relative + absolute + transform 针对未知宽高的元素

    .wrap {
      width: 300px;
      height: 300px;
      background: yellow;
      /*加颜色是为了看界面效果*/
      position: relative;
    }

    .inner {
      position: absolute;
      top: 50%;
      left: 50%;
      transform: translate(-50%, -50%);
      background: orange;
      /*加颜色是为了看界面效果*/
    }

  弹性布局 display flex 

    .wrap {
      width: 300px;
      height: 300px;
      background: yellow;
      /*加颜色是为了看界面效果*/
      display: flex;
      justify-content: center; /*水平居中*/
      align-items: center; /*垂直居中*/
    }
    .inner {
      background: orange;
    }

表格布局 display table cell

    .wrap {
      width: 300px;
      height: 300px;
      background: yellow;
      /*加颜色是为了看界面效果*/
      display: table-cell;
      text-align: center;
      vertical-align: middle;
    }
    .inner {
      display: inline-block;
      background: orange;
    }