一个宽度不确定的DIV里放三个水平对齐的DIV，左右两个DIV宽度固定为100px，中间那个DIV自适应宽度

方法一：浮动注意三个div的位置

<html>
<head>
<meta charset="utf-8">
<style type="text/css">
  *{
   margin:0;
   padding:0;
  }
  .left{
    100px;
   background-color: red;
   height:100%;
   float:left;
  }
  .middle{
   auto;
   height:100%;
   background-color: yellow;
  }
  .right{
   100px;
   background-color: blue;
   float:right;
   height:100%;
  }

</style>
</head>
<body>
<div id="id">
  <div class="left"></div>
  <div class="right"></div>
  <div class="middle"></div>
</div>
</body>
</html>

方法二：定位

<html>
<head>
<meta charset="utf-8">
<style type="text/css">
  *{
   margin:0;
   padding:0;
  }
  #id{
   position: relative;
  }
  .left{
    100px;
   background-color: red;
   height:100%;
   position: absolute;
   top:0;
   left:0;
  }
  .middle{
   auto;
   height:100%;
   background-color: yellow;
   margin:0 100px;
  }
  .right{
   100px;
   background-color: blue;
   height:100%;
   position: absolute;
   top:0;
   right:0;
  }

</style>
</head>
<body>
<div id="id">
  <div class="left"></div>
  <div class="middle"></div>
  <div class="right"></div>
</div>
</body>
</html>