@charset "UTF-8";

body {
  margin: 0;
  background: #e0e0e0;
  text-align: center;
  font-family: Verdana, sans-serif;
  color: #fff;
}

#btn {
  width: 200px;
  margin: 0 auto;
  padding: 7px;
  border-radius: 5px;
  background: #00aaff;
  box-shadow: 0 4px 0 #0088cc;
  cursor: pointer;
}

  #btn:hover {
    opacity: 0.8;
  }

  #card {
    margin: 60px auto 20px;
    width: 400px;
    height: 100px;
    cursor: pointer;
    font-size: 32px;
    line-height: 100px;
    perspective: 100px;
    transform-style: preserve-3d;
    transition: transform .8s;
  }

  #card-front,
  #card-back {
    display: block;
    width: 100%;
    height: 100%;
    border-radius: 5px;
    position: absolute;
    backface-visibility: hidden;
  }

  #card-front {
    background: #fff;
    color: #333;
  }

  #card-back {
    background: #00aaff;
    transform: rotateY(180deg);
  }

  .open {
    transform: rotateY(180deg);
  }