<div class="test">
<div class="img"></div>
</div>
.test {
width: 800px;
height: 200px;
border: 10px solid #bebebe;
border-bottom: none;
margin: 100px auto;
position: relative;
}
.test:after {
content: '';
display: block;
width: 100%;
height: 10px;
background: #bebebe;
position: absolute;
left: 0;
bottom: 0;
}
.img {
width: 200px;
height: 400px;
background: yellow;
margin: -100px auto 0;
}