用css写三角

<div id="demo"></div>

#demo{
  width:100px;
  height:100px;
  background-color:#fff;
  border:2px solid #000;
  position:relative;
}
#demo:after,#demo:before{
  position:absolute;
  width:0;
  height:0;
  left:100%;
  border:solid transparent;
  content:"";
}
#demo:after{
  top:20px;
  border-width:10px;
  border-left-color:#fff;
}
#demo:before{
  top:18px;
  border-width:12px;
  border-left-color:#000;
}