参考回答:
三角形原理:边框的均分原理
div { width:0px; height:0px; border-top:10px solid red; border-right:10px solid transparent; border-bottom:10px solid transparent; border-left:10px solid transparent;}

参考回答:
三角形原理:边框的均分原理
div { width:0px; height:0px; border-top:10px solid red; border-right:10px solid transparent; border-bottom:10px solid transparent; border-left:10px solid transparent;}