* {box-sizing: border-box;}
body {font-family: Arial, Helvetica, sans-serif;}

header {background-color: rgb(0,0,153);
        padding: 30px ;
        text-align: center;
        font-size: 35px;
        color: black;}

nav {float: left;
     width: 30%;
     height: 2500px;
     background: rgb(0,102,255);
     padding: 20px ;}
     
nav ul {list-style-type: none;
       padding: 0 ;}
       
article {float: left;
        padding: 20px ;
        width: 70%;
        background-color:rgb(102,204,255);
        height: 2500px;}
       
section::after {content: "";display:
               table;
               clear: both;}
               
footer {background-color: rgb(0,153,255);
       padding: 10px ;
       text-align: center;
       color: black;}
       
@media (max-width: 600px;) {
nav, article{width: 100%; height: auto;}