html {
    height: 100%;
    font-family: 'Courier', monospace;
}
header, footer {
    flex: none;
}
body {
    background-color: #f4f4f4;
    display: flex;
    flex-direction: column;
    height: 100%;
     width: 90%;        
  max-width: 1200px;  
  margin: 0 auto;     
}
main {
    overflow-y: auto;
    flex: auto;
}
.topnav {
  background-color: black;
  overflow: hidden;
}
.topnav a {
  float: left;
  color: #f2f2f2;
  text-align: justify;
  padding: 14px 16px;
  text-decoration: none;
  font-size: 20px;
}