SOURCE

console 命令行工具 X clear

                    
>
console
<header>
  <div class="head-left"></div>
</header>
<div class="mainbox">
  <div class="main-left"></div>
</div>
header {
  height: 50px;
  background: yellow;
}
.head-left {
  float: left;
  height: 50px;
  width: 200px;
  background: green;
}
.mainbox {
  position: relative;
  width: 500px;
  height: 300px;
  margin: -20px auto 0;
  background: #39f;
  z-index: 2;
}
.main-left {
  float: left;
  width: 50%;
  height: 50px;
  background: red;
}