CSS画三角

不追求三角形描边

&::after {
  position: absolute;
  content: "";
  display: block;
  width: 0;
  height: 0;
  border: 10px solid;
  border-color: transparent transparent transparent #fff ;
  top: 30px;
  right: -21px;
}

追求描边

        width: 10px;
        height: 10px;
        border: 1px solid red;
        transform: rotate(45deg);
        border-left: none;
        border-bottom: none;