css实现三角效果

demo
<!DOCTYPE html>
<html lang="zh">
<head>
<meta charset=utf-8>
<title>demo</title>
</head>

<style>
#demo{
  width:100px; 
  height:100px; 
  border:2px solid #000;
}
#demo:before{
  content:''; 
  display:block; 
  width:0; 
  height:0; 
  position:relative; 
  top:10px; 
  left:100px; 
  border-left:9px solid #000; 
  border-top:7px solid transparent; 
  border-bottom:7px solid transparent;
}
#demo:after{
  content:''; 
  display:block; 
  width:0; 
  height:0; 
  position:relative; 
  top:-2px; 
  left:100px; 
  border-left:7px solid #fff; 
  border-top:5px solid transparent; 
  border-bottom:5px solid transparent;
}
#s{
  width:0; 
  height:0; 
  border-left:20px solid transparent;       /*→*/
  border-right:20px solid transparent;      /*←*/
  border-top:20px solid transparent;        /*↓*/
  border-bottom:20px solid green;           /*↑*/
}
</style>
<body>
   <div id='demo'></div>


   <div id="s"></div>
</body>  
</html>