width: 0;
    height: 0;
    vertical-align: middle;
    border-top: 4px solid #aaa;
//左右透明
    border-right: 4px solid transparent;
    border-left: 4px solid transparent;

或者

<style>
    .triangle{
        border: 10px solid transparent;
        border-top:10px solid red;
        font-size: 0;
    }
</style>
<span class="triangle"></span>

5DF(4V749HVM)X`V@EE$ZJI.png

• transparent 用来指定全透明色彩
• 兼容性IE9+