CSS能够生成各种形状。正方形和矩形很容易，因为它们是 web 的自然形状。添加宽度和高度，就得到了所需的精确大小的矩形。添加边框半径，你就可以把这个形状变成圆形，足够多的边框半径，你就可以把这些矩形变成圆形和椭圆形。

我们还可以使用 CSS 伪元素中的 ::before 和 ::after，这为我们提供了向原始元素添加另外两个形状的可能性。通过巧妙地使用定位、转换和许多其他技巧，我们可以只用一个 HTML 元素在 CSS 中创建许多形状。

虽然我们现在大都使用字体图标或者svg图片，似乎使用 CSS 来做图标意义不是很大，但怎么实现这些图标用到的一些技巧及思路是很值得我们的学习。

1.正方形

  #square {
      width: 100px;
      height: 100px;
      background: red;
    }

2.长方形

 #rectangle {
      width: 200px;
      height: 100px;
      background: red;
    }

3.圆形

#circle {
      width: 100px;
      height: 100px;
      background: red;
      border-radius: 50%
    }

4.椭圆形

#oval {
      width: 200px;
      height: 100px;
      background: red;
      border-radius: 100px / 50px;
    }

5.上三角

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

6.下三角

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

7.左三角

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

8.右三角

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

9.左上角

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

10.右上角

 #triangle-topright {
      width: 0;
      height: 0;
      border-top: 100px solid red;
      border-left: 100px solid transparent;
    }

11.左下角

#triangle-bottomleft {
      width: 0;
      height: 0;
      border-bottom: 100px solid red;
      border-right: 100px solid transparent;
    }

12.右下角

 #triangle-bottomright {
      width: 0;
      height: 0;
      border-bottom: 100px solid red;
      border-left: 100px solid transparent;
    }

13.箭头

  #curvedarrow {
      position: relative;
      width: 0;
      height: 0;
      border-top: 9px solid transparent;
      border-right: 9px solid red;
      transform: rotate(10deg);
    }
    #curvedarrow:after {
      content: "";
      position: absolute;
      border: 0 solid transparent;
      border-top: 3px solid red;
      border-radius: 20px 0 0 0;
      top: -12px;
      left: -9px;
      width: 12px;
      height: 12px;
      transform: rotate(45deg);
    }

14.梯形

 #trapezoid {
      border-bottom: 100px solid red;
      border-left: 25px solid transparent;
      border-right: 25px solid transparent;
      height: 0;
      width: 100px;
    }

15.平行四边形

#parallelogram {
      width: 150px;
      height: 100px;
      transform: skew(20deg);
      background: red;
    }

16.星星 (6角)

#star-six {
      width: 0;
      height: 0;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      border-bottom: 100px solid red;
      position: relative;
    }
    #star-six:after {
      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;
    }

17.星星 (5角)

   #star-five {
      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 {
      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 {
      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: '';
    }

18.五边形

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

19.六边形

 #hexagon {
      width: 100px;
      height: 55px;
      background: red;
      position: relative;
    }
    #hexagon:before {
      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 {
      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;
    }

20.八边形

 #octagon {
      width: 100px;
      height: 100px;
      background: red;
      position: relative;
    }
    #octagon:before {
      content: "";
      width: 100px;
      height: 0;
      position: absolute;
      top: 0;
      left: 0;
      border-bottom: 29px solid red;
      border-left: 29px solid #eee;
      border-right: 29px solid #eee;
    }
    #octagon:after {
      content: "";
      width: 100px;
      height: 0;
      position: absolute;
      bottom: 0;
      left: 0;
      border-top: 29px solid red;
      border-left: 29px solid #eee;
      border-right: 29px solid #eee;
    }

21.爱心

 #heart {
      position: relative;
      width: 100px;
      height: 90px;
    }
    #heart:before,
    #heart:after {
      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%;
    }

22.无穷大

 #infinity {
      position: relative;
      width: 212px;
      height: 100px;
      box-sizing: content-box;
    }
    #infinity:before,
    #infinity:after {
      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);
    }

23.菱形

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

24.钻石

    #diamond-shield {
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-bottom: 20px solid red;
      position: relative;
      top: -50px;
    }
    #diamond-shield:after {
      content: '';
      position: absolute;
      left: -50px;
      top: 20px;
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-top: 70px solid red;
    }

25.钻戒

    #diamond-narrow {
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-bottom: 70px solid red;
      position: relative;
      top: -50px;
    }
    #diamond-narrow:after {
      content: '';
      position: absolute;
      left: -50px;
      top: 70px;
      width: 0;
      height: 0;
      border: 50px solid transparent;
      border-top: 70px solid red;
    }

26.钻石2

#cut-diamond {
      border-style: solid;
      border-color: transparent transparent red transparent;
      border-width: 0 25px 25px 25px;
      height: 0;
      width: 50px;
      box-sizing: content-box;
      position: relative;
      margin: 20px 0 50px 0;
    }
    #cut-diamond:after {
      content: "";
      position: absolute;
      top: 25px;
      left: -25px;
      width: 0;
      height: 0;
      border-style: solid;
      border-color: red transparent transparent transparent;
      border-width: 70px 50px 0 50px;
    }

27.蛋蛋

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

28.吃豆人

 #pacman {
      width: 0px;
      height: 0px;
      border-right: 60px solid transparent;
      border-top: 60px solid red;
      border-left: 60px solid red;
      border-bottom: 60px solid red;
      border-top-left-radius: 60px;
      border-top-right-radius: 60px;
      border-bottom-left-radius: 60px;
      border-bottom-right-radius: 60px;
    }

29.对话泡泡

 #talkbubble {
      width: 120px;
      height: 80px;
      background: red;
      position: relative;
      -moz-border-radius: 10px;
      -webkit-border-radius: 10px;
      border-radius: 10px;
    }
    #talkbubble:before {
      content: "";
      position: absolute;
      right: 100%;
      top: 26px;
      width: 0;
      height: 0;
      border-top: 13px solid transparent;
      border-right: 26px solid red;
      border-bottom: 13px solid transparent;
    }

30. 12点爆发

 #burst-12 {
      background: red;
      width: 80px;
      height: 80px;
      position: relative;
      text-align: center;
    }
    #burst-12:before,
    #burst-12:after {
      content: "";
      position: absolute;
      top: 0;
      left: 0;
      height: 80px;
      width: 80px;
      background: red;
    }
    #burst-12:before {
      transform: rotate(30deg);
    }
    #burst-12:after {
      transform: rotate(60deg);
    }

31. 8点爆发

#burst-8 {
      background: red;
      width: 80px;
      height: 80px;
      position: relative;
      text-align: center;
      transform: rotate(20deg);
    }
    #burst-8:before {
      content: "";
      position: absolute;
      top: 0;
      left: 0;
      height: 80px;
      width: 80px;
      background: red;
      transform: rotate(135deg);
    }

32.太极

 #yin-yang {
      width: 96px;
      box-sizing: content-box;
      height: 48px;
      background: #eee;
      border-color: red;
      border-style: solid;
      border-width: 2px 2px 50px 2px;
      border-radius: 100%;
      position: relative;
    }
    #yin-yang:before {
      content: "";
      position: absolute;
      top: 50%;
      left: 0;
      background: #eee;
      border: 18px solid red;
      border-radius: 100%;
      width: 12px;
      height: 12px;
      box-sizing: content-box;
    }
    #yin-yang:after {
      content: "";
      position: absolute;
      top: 50%;
      left: 50%;
      background: red;
      border: 18px solid #eee;
      border-radius: 100%;
      width: 12px;
      height: 12px;
      box-sizing: content-box;
    }

33.徽章丝带

 #badge-ribbon {
      position: relative;
      background: red;
      height: 100px;
      width: 100px;
      border-radius: 50px;
    }
    #badge-ribbon:before,
    #badge-ribbon:after {
      content: '';
      position: absolute;
      border-bottom: 70px solid red;
      border-left: 40px solid transparent;
      border-right: 40px solid transparent;
      top: 70px;
      left: -10px;
      transform: rotate(-140deg);
    }
    #badge-ribbon:after {
      left: auto;
      right: -10px;
      transform: rotate(140deg);
    }

34.太空入侵者（电脑游戏名）

#space-invader {
      box-shadow: 0 0 0 1em red,
      0 1em 0 1em red,
      -2.5em 1.5em 0 .5em red,
      2.5em 1.5em 0 .5em red,
      -3em -3em 0 0 red,
      3em -3em 0 0 red,
      -2em -2em 0 0 red,
      2em -2em 0 0 red,
      -3em -1em 0 0 red,
      -2em -1em 0 0 red,
      2em -1em 0 0 red,
      3em -1em 0 0 red,
      -4em 0 0 0 red,
      -3em 0 0 0 red,
      3em 0 0 0 red,
      4em 0 0 0 red,
      -5em 1em 0 0 red,
      -4em 1em 0 0 red,
      4em 1em 0 0 red,
      5em 1em 0 0 red,
      -5em 2em 0 0 red,
      5em 2em 0 0 red,
      -5em 3em 0 0 red,
      -3em 3em 0 0 red,
      3em 3em 0 0 red,
      5em 3em 0 0 red,
      -2em 4em 0 0 red,
      -1em 4em 0 0 red,
      1em 4em 0 0 red,
      2em 4em 0 0 red;
      background: red;
      width: 1em;
      height: 1em;
      overflow: hidden;
      margin: 50px 0 70px 65px;
    }

35.电视

#tv {
      position: relative;
      width: 200px;
      height: 150px;
      margin: 20px 0;
      background: red;
      border-radius: 50% / 10%;
      color: white;
      text-align: center;
      text-indent: .1em;
    }
    #tv:before {
      content: '';
      position: absolute;
      top: 10%;
      bottom: 10%;
      right: -5%;
      left: -5%;
      background: inherit;
      border-radius: 5% / 50%;
    }

36.雪佛龙

#chevron {
      position: relative;
      text-align: center;
      padding: 12px;
      margin-bottom: 6px;
      height: 60px;
      width: 200px;
    }
    #chevron:before {
      content: '';
      position: absolute;
      top: 0;
      left: 0;
      height: 100%;
      width: 51%;
      background: red;
      transform: skew(0deg, 6deg);
    }
    #chevron:after {
      content: '';
      position: absolute;
      top: 0;
      right: 0;
      height: 100%;
      width: 50%;
      background: red;
      transform: skew(0deg, -6deg);
    }

37.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);
    }

38.Facebook图标

 #facebook-icon {
      background: red;
      text-indent: -999em;
      width: 100px;
      height: 110px;
      box-sizing: content-box;
      border-radius: 5px;
      position: relative;
      overflow: hidden;
      border: 15px solid red;
      border-bottom: 0;
    }
    #facebook-icon:before {
      content: "/20";
      position: absolute;
      background: red;
      width: 40px;
      height: 90px;
      bottom: -30px;
      right: -37px;
      border: 20px solid #eee;
      border-radius: 25px;
      box-sizing: content-box;
    }
    #facebook-icon:after {
      content: "/20";
      position: absolute;
      width: 55px;
      top: 50px;
      height: 20px;
      background: #eee;
      right: 5px;
      box-sizing: content-box;
    }

39.月亮

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

40.旗

 #flag {
      width: 110px;
      height: 56px;
      box-sizing: content-box;
      padding-top: 15px;
      position: relative;
      background: red;
      color: white;
      font-size: 11px;
      letter-spacing: 0.2em;
      text-align: center;
      text-transform: uppercase;
    }
    #flag:after {
      content: "";
      position: absolute;
      left: 0;
      bottom: 0;
      width: 0;
      height: 0;
      border-bottom: 13px solid #eee;
      border-left: 55px solid transparent;
      border-right: 55px solid transparent;
    }

41.圆锥

 #cone {
      width: 0;
      height: 0;
      border-left: 70px solid transparent;
      border-right: 70px solid transparent;
      border-top: 100px solid red;
      border-radius: 50%;
    }

42.十字架

   #cross {
      background: red;
      height: 100px;
      position: relative;
      width: 20px;
    }
    #cross:after {
      background: red;
      content: "";
      height: 20px;
      left: -40px;
      position: absolute;
      top: 40px;
      width: 100px;
    }

43.根基

 #base {
      background: red;
      display: inline-block;
      height: 55px;
      margin-left: 20px;
      margin-top: 55px;
      position: relative;
      width: 100px;
    }
    #base:before {
      border-bottom: 35px solid red;
      border-left: 50px solid transparent;
      border-right: 50px solid transparent;
      content: "";
      height: 0;
      left: 0;
      position: absolute;
      top: -35px;
      width: 0;
    }

44.指示器

#pointer {
      width: 200px;
      height: 40px;
      position: relative;
      background: red;
    }
    #pointer:after {
      content: "";
      position: absolute;
      left: 0;
      bottom: 0;
      width: 0;
      height: 0;
      border-left: 20px solid white;
      border-top: 20px solid transparent;
      border-bottom: 20px solid transparent;
    }
    #pointer:before {
      content: "";
      position: absolute;
      right: -20px;
      bottom: 0;
      width: 0;
      height: 0;
      border-left: 20px solid red;
      border-top: 20px solid transparent;
      border-bottom: 20px solid transparent;
    }

45.锁

#lock {
  font-size: 8px;
  position: relative;
  width: 18em;
  height: 13em;
  border-radius: 2em;
  top: 10em;
  box-sizing: border-box;
  border: 3.5em solid red;
  border-right-width: 7.5em;
  border-left-width: 7.5em;
  margin: 0 0 6rem 0;
}
#lock:before {
  content: "";
  box-sizing: border-box;
  position: absolute;
  border: 2.5em solid red;
  width: 14em;
  height: 12em;
  left: 50%;
  margin-left: -7em;
  top: -12em;
  border-top-left-radius: 7em;
  border-top-right-radius: 7em;
}
#lock:after {
  content: "";
  box-sizing: border-box;
  position: absolute;
  border: 1em solid red;
  width: 5em;
  height: 8em;
  border-radius: 2.5em;
  left: 50%;
  top: -1em;
  margin-left: -2.5em;
}