css绘制三角形

#triangle-down {
width: 0;
height: 0;
border-left: 0px solid transparent;
border-bottom: 50px solid transparent;
border-right: 500px solid red;
border-top: 50px solid transparent;
}
<div id="triangle-down"></div>

image.png

---

可拓展为梯形等等