the css border's future is "45° between border and border"

take off border-top,and set twice bigger width on the border-bottom
set the div's height and width = 0
last,set the border-right, border-left's color = transparent

 div {
 width: 0px;
 height: 0px;
 border-right: 30px solid transparent;
 border-left: 30px solid transparent;
 border-bottom: 60px solid #c1d7ae;


450 0 1