SOURCE

<div class="outer">
	
</div>

.outer {
    width: 500px;
    height: 100px;
    border: 2px solid red;
    position: relative;
    background: black;
}
.outer::before {
    position: absolute;
    content: '';
    display: block;
    width: 20px;
    height: 20px;
    background: black;
    bottom: -12px;
    margin: 0 auto;
    left: 0;
    right: 0;
    border: 2px solid red;
    transform: rotate(45deg);
    border-color: transparent red red transparent;
}