main {
  max-width: 50em;
  margin: 5% auto;
}
footer{
  margin: 0 auto;
}
p { text-align: center; }
p.ref { 
  text-align: right;
  font-size: small;
  font-family: sans-serif;
}
img.mu { 
  display: block;
  margin: 0 auto;
}
p.mu {
  color: gray;
  text-align: center;
  font-size: small;
}
p.footer {
  color: gray;
  text-align: center;
  font-size: small;
}
div.mu {
  display: block;
  margin: 10%;
}
div.heading {
  margin: 0 auto;
} 

.tab{ 
  max-width: 45em;
  min-height: 20em;
  margin: 10% auto;
  position: relative;
}
@media screen {
  .tab > div { display: inline; }
  .tab > div > a {
    margin-left: -1px;
    position: relative;
    left: 1px;
    text-decoration: none;
    color: black;
    background: white;
    display: block;
    float: left;
    padding: 5px 10px;
    border: 1px solid #ccc;
    border-bottom: 1px solid white; 
  }
  .tab > div:not(:target) > a { 
    border-bottom: 0;
    background: #eee;
  }
  .tab > div:target > a { background: white; }
  .tab > div > div { 
    background: white;
    z-index: -2;
    left: 0;
    top: 30px;
    bottom: 0;
    right: 0; 
    padding: 20px;
    border: 1px solid #ccc;
  }
  .tab > div:not(:target) > div { position: absolute }
  .tab > div:target > div { 
    position: absolute;
    z-index: 0; 
  }
}
@media print {
  .tab > div {
    border: 1px solid #ccc;
    margin 5%;
  }
  .tab > div > a {
      color: #ccc;
      text-decoration: none;
  }
}