ul {
    list-style-type: none;
    background-color: gray;
    margin: 0;
    padding: 0;
    overflow: hidden;
}

li {
    float: left;
}

li a {
    text-decoration: none;
    text-align: center;
    color: white;
    display: inline-block;
    padding: 14px 16px;
}

#cats {
    display: flex;
    flex-direction: column;
    align-items: center;
}

li a:hover {
    background-color: black;
}

img {
    float: right;
}

span {
    float: left;
    width: 0.7em;
    font-size: 400%;
    font-family: algerian, courier;
    line-height: 80%;
}

#learn {
    float: left;
    background-color: aqua;
    width: 400px;
    height: 200px;
    overflow: auto;
}

#geeks {
    float: left;
    background-color: aqua;
    width: 400px;
    height: 200px;
    overflow: auto;
}

#contribute {
    float: left;
    background-color: aqua;
    width: 400px;
    height: 200px;
    overflow: clip;
}

*{box-sizing: border-box;}
  
body {
    font-family: Arial, Helvetica, sans-serif;
}
  
header {
    background-color: #666;
    padding: 30px;
    text-align: center;
    font-size: 35px;
    color: white;
}
  
nav {
    float: left;
    width: 30%;
    background: #ccc;
    padding: 20px;
    display: flex;
    flex-direction: column;
    align-items: center;
}
  
nav ul {
    list-style-type: none;
    padding: 0;
}
  
article {
    float: left;
    padding: 20px;
    width: 70%;
    background-color: #f1f1f1;
}
  
section::after {
    content: "";
    display: table;
    clear: both;
}

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

footer {
    color: white;
    background-color: grey;
    padding: 15px;
    text-align: center;
}