Edit in JSFiddle


              
            
          
            
              
                
<div class="mainDiv"></div>
.mainDiv {
            width: 200px;
            height: 50px;
            border: 5px solid red;
            position: relative;
        }

        .mainDiv:after {
            content: '';
            display: block;
            position: absolute;
            width: 100px;
            top: -5px;
            left: 0px;
            border: 5px solid white;
        }