左右宽度固定，中间宽度自适应 - 《H5C3 JS 教程》

使用浮动

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
      .contain{
          width: 100%;
          height: 500px;
      }
      .left{
          width: 250px;
          float: left;
          height: 100%;
          background: pink;
      }
      .right{
          width: 250px;
          float: right;
          height: 100%;
          background: plum;
      }
      .center{
          width: auto;
          height: 100%;
          background: palegoldenrod;
      }
  </style>
</head>
<body>
<div class="contain">
  <div class="left">left</div>
  <div class="right">right</div>
  <div class="center">center</div>
</div>
</body>
</html>

使用 flex，自适应的元素设置为 flex: 1; 固定宽度设置固定宽度即可，父元素设置 flex 弹性布局

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
      .flex{
          display: flex;
      }
      .left{
          width: 200px;
          background: darkseagreen;
      }
      .center{
          flex: 1;
          background: #ff9db5;
      }
      .right{
          width: 200px;
          background: purple;
      }
  </style>
</head>
<body>
<div class="flex">
  <div class="left">left</div>
  <div class="center">center</div>
  <div class="right">right</div>
</div>
</body>
</html>