绘制梯形
width:10px; border-top:10px solid red; border-right:10px solid transparent; border-left:10px solid transparent; border-bottom:10px solid transparent;
绘制三角形:
width:0px; border-top:10px solid red; border-right:10px solid transparent; border-left:10px solid transparent; border-bottom:10px solid transparent;
绘制直角三角形:
width:0px; border-top:20px solid red; border-right:0px solid transparent; border-left:10px solid transparent; border-bottom:10px solid transparent;