<div class='container'>
<div class="c_left"></div>
<div class="c_right"></div>
</div>
.container{
width:800px;
height:400px;
background:#708cfe;
margin:30px auto;
border:1px solid #000;
border-radius:20px;
}
.c_left{width:35%;height:100%;background:#fff;border-radius:20px 0 0 20px;}