<html> <head> <style> .up { height:0; width:0; border-top:20px solid black; border-left:20px solid transparent; transform:rotate(-45deg); } .down { height:0; width:0; border-bottom:20px solid black; border-right:20px solid transparent; transform:rotate(-45deg); } .left { height:0; width:0; border-bottom:20px solid black; border-right:20px solid transparent; transform:rotate(45deg); } .right { height:0; width:0; border-top:20px solid black; border-left:20px solid transparent; transform:rotate(45deg); } </style> </head> <body> <div class="up"></div> </body> </html>
原文：http://www.cnblogs.com/pumushan/p/4702530.html