/* 正三角 */
.up-triangle {
   width: 0;
   height: 0;
   border-style: solid;
   border-width: 0 25px 40px 25px;
   border-color: transparent transparent rgb(245, 129, 127) transparent;
 }

 /* 倒三角 */
 .down-triangle {
   width: 0;
   height: 0;
   border-style: solid;
   border-width: 40px 25px 0 25px;
   border-color:  rgb(245, 129, 127) transparent transparent transparent;
 }
 div:last-child {
   margin-top: 1rem;
 }

纯CSS绘制三角形_CSS image.png