<div class="inner"></div>
<div class="wrapp"></div>

css 



   body {
    padding:8px;
}
div.wrapp {
    width:300px;
    height:300px;
    border:2px solid green;
    margin:auto;
    position:relative;
}
div.wrapp:before {
    content:'';
    position:absolute;
    width:100%;
    height:1px;
    right:0;
    top:-6px;
    background:blue;
    z-index:1;

}
.inner {
    width:50%;
    float:left;
    position:absolute;
    height:1px;
    left:0;
    top:12px;
     background:blue;


}