nav {
    float: left;
  width: 20%;
  background: #e4e4e4;
  padding: 20px;
}

main   {
    padding: 20px;
    width: 100%;
    background-color: #f1f1f1;
}


section::after {
    content: "";
    display: table;
    clear: both;
}

@media (max-width: 600px) {
    nav, article {
      width: 100%;
      height: auto;
    }
}