HTML에서 요소를 중앙에 배치할 때 무엇에 주의해야 합니까?

.first{
    width: 300px;
    height: 100px;
    background-color: black;
    color: white;
    text-align: center;
    margin: 0 auto; //针对块级元素
  }

<div class="first">
不使用定位（1）
</div>

.second{
    width: 300px;
    height: 100px;
    background-color: green;
  }
  .s_child{
    width: 150px;
    line-height: 100px;
  }
<div class="second">    
<div class="s_child">   
   不使用定位（2）    
</div> 
</div>

.dw_one{
      width: 600px;
      height: 300px;
      position: absolute;
      background: black;
    }
    .dw_one_child{
      background: white;
      position: relative;
      width: 50px;
      height: 50px;
      top: 50%;
      left: 50%;
      margin-top: -25px;
      margin-left: -25px;
    }
    <div class="dw_one">
    <div class="dw_one_child">
      a    
    </div>
    </div>

.wrapper{
      width: 600px;
      height: 600px;
    }
    .dw_two{
      width: 100%;
      height: 100%;
      position: absolute;
      background: black;
    }
    .dw_two_child{
      background: white;
      position: relative;
      top: 50%;
      left: 50%;
      width: 100px;
      height: 100px;
      transform:translate(-50%, -50%);
      -moz-transform:translate(-50%, -50%);
      -ms-transform:translate(-50%, -50%);
      -o-transform:translate(-50%, -50%);
      -webkit-transform:translate(-50%, -50%);
    }
    <div class="wrapper">
  <div class="dw_two">
      <div class="dw_two_child">
        a
      </div>
  </div>
</div>

#outer{
           width: 200px;
           height: 200px;
           background: #cccccc;
           display: table;
           _position: relative;   // "_"为了兼容IE6
       }
       #inner{
           display: table-cell;
           vertical-align: middle;
           _position: absolute;
           _top: 50%;
       }
       #content{
           _position: relative;
           _top: -50%;
       }
<div id="outer">
     <div id="inner">
       <div id="content">
          Paradise_追逐者所写的居中问题总结之我见
       </div>
    </div>
</div>