<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;
}