style.css

#coin {
    position: relative;
    margin: 0 auto;
    width: 100px;
    height: 100px;
  }
  #coin div {
    width: 100%;
    height: 100%;
    -webkit-border-radius: 50%;
    -moz-border-radius: 50%;
    border-radius: 50%;
    -webkit-box-shadow: inset 0 0 45px rgba(255, 255, 255, 0.3),
      0 12px 20px -10px rgba(0, 0, 0, 0.4);
    -moz-box-shadow: inset 0 0 45px rgba(255, 255, 255, 0.3),
      0 12px 20px -10px rgba(0, 0, 0, 0.4);
    box-shadow: inset 0 0 45px rgba(255, 255, 255, 0.3),
      0 12px 20px -10px rgba(0, 0, 0, 0.4);
  }
  .side-a {
    background-color: #bb0000;
    color: white;
    text-align: center;
  }
  .side-b {
    background-color: #3e3e3e;
    color: white;
    text-align: center;
  }
  
  #coin {
    transition: -webkit-transform 1s ease-in;
    -webkit-transform-style: preserve-3d;
  }
  #coin div {
    position: absolute;
    -webkit-backface-visibility: hidden;
  }
  .side-a {
    z-index: 100;
  }
  .side-b {
    -webkit-transform: rotateY(-180deg);
  }
  
  #coin.heads {
    -webkit-animation: flipHeads 2s ease-out forwards;
    -moz-animation: flipHeads 2s ease-out forwards;
    -o-animation: flipHeads 2s ease-out forwards;
    animation: flipHeads 2s ease-out forwards;
  }
  #coin.tails {
    -webkit-animation: flipTails 2s ease-out forwards;
    -moz-animation: flipTails 2s ease-out forwards;
    -o-animation: flipTails 2s ease-out forwards;
    animation: flipTails 2s ease-out forwards;
  }
  
  @-webkit-keyframes flipHeads {
    from {
      -webkit-transform: rotateY(0);
      -moz-transform: rotateY(0);
      transform: rotateY(0);
    }
    to {
      -webkit-transform: rotateY(540deg);
      -moz-transform: rotateY(540deg);
      transform: rotateY(540deg);
    }
  }
  @-webkit-keyframes flipTails {
    from {
      -webkit-transform: rotateY(0);
      -moz-transform: rotateY(0);
      transform: rotateY(0);
    }
    to {
      -webkit-transform: rotateY(720deg);
      -moz-transform: rotateY(720deg);
      transform: rotateY(720deg);
    }
  }