三角形/梯形

        三角形

width: 0;
height: 0;
border-top: 40px solid transparent;
border-left: 40px solid transparent;
border-right: 40px solid transparent;
border-bottom: 40px solid #ff0000;

        梯形

width: 50px;
height: 0;
border-bottom: 50px solid red;
border-left: 50px solid #fff;
border-right: 50px solid #fff;

 一边固定另一边自适应