水平与垂直对齐

实例代码：


<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>元素的水平与垂直对齐</title>
</head>
<style>
    *{
        margin: 0;
        padding: 0;
        box-sizing: border-box;
    }
  .box{
       width: 300px;
       height: 300px;
       margin: 5px;      
       border: 1px solid #000;
       position: relative;
       float: left;
   }
  .item{         
       width: 100px;
       height: 100px;       
       border: 1px solid #000;
       position: absolute;
       top: 0;    
       left: 0;
       right: 0;
       bottom: 0;
       margin:auto;
  }
  .item_1{
      text-align: center;
  }
    .box1{
       width: 300px;
       height: 300px;
       margin: 5px;            
       border: 1px solid #000;
       float: left;
   }
  .box1 .item1{
      width: 100px;
      height: 100px;      
      border: 1px solid #000;
      text-align: center;
      margin: 0 auto;
  }
  .box2{
       width: 300px;
       height: 300px;
       margin: 5px;            
       border: 1px solid #000;
       float: left;
       position: relative;
  }
  .box2 .item2{
      width: 100px;
      height: 100px;      
      border: 1px solid #000;
      text-align: center;        
      position: absolute;  
      top: 100PX;
  }
</style>
<body>
   <div class="box">
       <div class="item"></div>      
       <div class="item_1">水平垂直居中</div>      
   </div>
   <div class="box1">
       <div class="item1"></div>       
       <div class="item1_1">水平居中</div>     
   </div>
   <div class="box2">
       <div class="item2"></div>
       <div class="item2_1">垂直居中</div>
   </div>
</body>
</html>