透明箭头/三角形缩进
.barShow { background-color: #000; opacity: 0.5;}.barShow:before { top: 0%; left: 50%; border: solid transparent; content: " "; height: 0; width: 0; position: absolute; pointer-events: none; border-top-color: #999; border-width: 20px; margin-left: -20px;}
<div class="barShow"></div>