SOURCE

<div class="box">闻道有先后，术业有专攻！</div>

.box{
    position: relative;
    left: 50px;
    padding: 0 20px;
    width: 380px;
    height: 80px;
    border-radius: 8px;
    background: #efefef;
    font-size: 18px;
    line-height: 80px;
}
.box:after{
    position: absolute;
    top: 50%;
    left: -15px;
    z-index: 1;
    display: block;
    margin-top: -15px;
    width: 0;
    border-color: transparent #efefef;
    border-style: solid;
    border-width: 15px 15px 15px 0;
    content: "";
}