纯css画三角形

1.通过设定边框来实现

<div class="demo"></div>
 
.demo {
        width: 0;
        height: 0;
        border: 50px solid transparent;
        border-top: 50px solid black;
        margin: 0 auto;
        /* border-bottom: 50px solid black; */
        /* border-left: 100px solid black; */
        /* border-left: 50px solid red;
           border-bottom: 50px solid red; */
      }

　　通过设置边框透明,三角形的颜色可以得到不同方向的,不同颜色的三角形.对应得到如下效果:

2.利用伪元素

<div class="demo"></div><br>.demo {
        margin: 100px auto;
        width: 200px;
        height: 100px;
        border: 1px solid black;
        position: relative;
        border-radius:5px;
      }
 
      .demo::after {
        content: "";
        position: absolute;
        left: -20px;
        top: 12px;
        width: 0;
        height: 0;
        border-top: 10px solid transparent;
        border-right: 10px solid #fff;
        border-left: 10px solid transparent;
        border-bottom: 10px solid transparent;
      }
 
      .demo::before {
        content: "";
        position: absolute;
        left: -22px;
        top: 11px;
        width: 0;
        height: 0;
        border-top: 11px solid transparent;
        border-right: 11px solid #000;
        border-left: 11px solid transparent;
        border-bottom: 11px solid transparent;
      }