<div class="box">
</div>
.box {
position: relative;
width: 600px;
height: 400px;
background: #fff;
border: 1px solid #ccc;
box-shadow: 2px 2px 2px #ccc;
margin:50px;
}
.box:after {
position: absolute;
display: inline-block;
top: 380px;
left: 300px;
width: 0;
height: 0px;
content: '';
border-style: solid;
border-width: 20px;
border-color: #fff #fff transparent transparent;
transform: rotate(135deg);
box-shadow: 2px -2px 2px #ccc;
}