https://css-tricks.com/the-shapes-of-css/
简单的正方形，圆形，椭圆形跳过。

1.三角形

    #triangle-top {
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 100px solid red;
    }
    #triangle-topleft {
      width: 0;
      height: 0;
      border-top: 100px solid red;
      border-right: 100px solid transparent;
    }

2.六芒星

    #star-six { /*正三角*/
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 100px solid red;
      position: relative;
    }
    #star-six:after { /*倒三角*/
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-top: 100px solid red;
      position: absolute;
      content: "";
      top: 30px;
      left: -50px;
    }

3.五角星

    #star-five {   /*五角星的左上&右下*/
      margin: 50px 0;
      position: relative;
      display: block;
      color: red;
      width: 0px;
      height: 0px;
      border-right: 100px solid transparent;
      border-bottom: 70px solid red;
      border-left: 100px solid transparent;
      transform: rotate(35deg);
    }
    #star-five:before {   /*五角星的上尖*/
      border-bottom: 80px solid red;
      border-left: 30px solid transparent;
      border-right: 30px solid transparent;
      position: absolute;
      height: 0;
      width: 0;
      top: -45px;
      left: -65px;
      display: block;
      content: '';
      transform: rotate(-35deg);
    }
    #star-five:after {   /*五角星的左下&右上*/
      position: absolute;
      display: block;
      color: red;
      top: 3px;
      left: -105px;
      width: 0px;
      height: 0px;
      border-right: 100px solid transparent;
      border-bottom: 70px solid red;
      border-left: 100px solid transparent;
      transform: rotate(-70deg);
      content: '';
    }

4.五边形

    #pentagon { /*下梯形*/
      position: relative;
      margin-top:50px;
      width: 54px;
      box-sizing: content-box;
      border-width: 50px 18px 0;
      border-style: solid;
      border-color: red transparent;
    }
    #pentagon:before { /*上三角*/
      content: "";
      position: absolute;
      height: 0;
      width: 0;
      top: -85px;
      left: -18px;
      border-width: 0 45px 35px;
      border-style: solid;
      border-color: transparent transparent red;
    }

5.六边形

    #hexagon { /*中方形*/
      margin-top:25px;
      width: 100px;
      height: 55px;
      background: red;
      position: relative;
    }
    #hexagon:before { /*上三角*/
      content: "";
      position: absolute;
      top: -25px;
      left: 0;
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 25px solid red;
    }
    #hexagon:after { /*下三角*/
      content: "";
      position: absolute;
      bottom: -25px;
      left: 0;
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-top: 25px solid red;
    }

6.爱心

    #heart { 
      position: relative;
      width: 100px;
      height: 90px;
    }
    #heart:before,
    #heart:after { /* 2个（长方形+半圆），旋转*/
      position: absolute;
      content: "";
      left: 50px;
      top: 0;
      width: 50px;
      height: 80px;
      background: red;
      border-radius: 50px 50px 0 0;
      transform: rotate(-45deg);
      transform-origin: 0 100%;
    }
    #heart:after {
      left: 0;
      transform: rotate(45deg);
      transform-origin: 100% 100%;
    }

7.无尽

 #infinity {
      position: relative;
      width: 212px;
      height: 100px;
      box-sizing: content-box;
    }
    #infinity:before,
    #infinity:after { /*同爱心*/
      content: "";
      box-sizing: content-box;
      position: absolute;
      top: 0;
      left: 0;
      width: 60px;
      height: 60px;
      border: 20px solid red;
      border-radius: 50px 50px 0 50px;
      transform: rotate(-45deg);
    }
    #infinity:after {
      left: auto;
      right: 0;
      border-radius: 50px 50px 50px 0;
      transform: rotate(45deg);
    }

8.菱形

    #diamond { /*上三角*/
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-bottom-color: red;
      position: relative;
      top: -50px;
    }
    #diamond:after {/*下三角*/
      content: '';
      position: absolute;
      left: -50px;
      top: 50px;
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-top-color: red;
    }

9.蛋

    #egg {
      display: block;
      width: 126px;
      height: 180px;
      background-color: red;
      border-radius: 50% 50% 50% 50% / 60% 60% 40% 40%;
    }

10.放大镜

    #magnifying-glass {
      font-size: 10em;
      display: inline-block;
      width: 0.4em;
      box-sizing: content-box;
      height: 0.4em;
      border: 0.1em solid red;
      position: relative;
      border-radius: 0.35em;
    }
    #magnifying-glass:before {
      content: "";
      display: inline-block;
      position: absolute;
      right: -0.25em;
      bottom: -0.1em;
      border-width: 0;
      background: red;
      width: 0.35em;
      height: 0.08em;
      transform: rotate(45deg);
    }

11.月亮

    #moon {
      width: 80px;
      height: 80px;
      border-radius: 50%;
      box-shadow: 15px 15px 0 0 red;
    }