css实现一个缺口小三角

.square{
     width:0;
     height:0;
     margin:0 auto;
     border:6px solid transparent;
     border-bottom: 6px solid red;  //自行调整方向,.
 } 

带缺口的三角形:

<div class="box b1"></div>
<div class="box b2"></div> 

.box{
    position: absolute;
    left: 0;
    top: 0;
    width: 0;
    height:0;
    border: 6px solid transparent;
}
.b1{
    border-bottom-color:#000 ;
}
.b2{
    border-bottom-color:#fff ;
}