左右經典佈局

左右兩列：一列寬度固定，一列寬度自適應

 * {
      box-sizing :border-box;
    }
    body {
      background-color: #ddd;
      text-align: center;
    }
    .main, .left {
      background-color: #fff;
    }
    .main {
      margin-left: 210px;
      float: left;
      width: calc(100% - 210px);
    }
    .left {
      float: left;
      width: 200px;
      margin-left: -100%;
    }
 ```
```html

 <div class="main">
    <p>this is main</p>
    <p>this is main</p>
    <p>this is main</p>
    <p>this is main</p>
    <p>this is main</p>
  </div>
  <div class="left">left</div>