空心三角形

.timeline .feed-unit:before,
.timeline .feed-unit:after{
    content: "";
    position: absolute;
    width: 0;
    height: 0;
}
.timeline .feed-unit:before{
    top:28px;
    left: -24px;
    border: 12px solid transparent;
    border-right: 12px solid #eee;
}
.timeline .feed-unit:after{
    top:30px;
    left: -20px;
    border: 10px solid transparent;
    border-right: 10px solid white;
}