* {
    margin: 0; padding: 0;
    border: 0; box-sizing: border-box;
}

body {
    width: 98vw; height: 100vh;
    background-color: crimson;
    font-family: sans-serif;
    margin: auto; overflow-x: hidden;
}
header {
    height: 10vh;
    background-color: darkblue;
}
nav {
    background-color: darkcyan;
}
nav ul {
    list-style-type: none;
}
nav ul li {
    border: 2px solid black;
    text-align: center; 
    width: 60vw; margin: auto;

}
nav ul li a {
    text-decoration: none;
}
section {
    background-color: orange;
    height: 60vh; 
    display: flex; flex-flow: row wrap;
}
div {
    flex-basis: 100%;
    border: solid 2px black;
    
}
footer {
    height: 10vh;
    background-color: darkorchid;
}

@media screen and (min-width: 720px) and (min-device-width: 720px) {
    
    div {
        flex: 1 0 50%;
        width: 100;
        border: solid 2px black;
    }
}

@media screen and (min-width: 1200px) and (min-device-width: 1200px) {
    nav ul {
        display: flex
    }
    nav ul li {
        width: 10vw;
    }
    section {
        height: 77%;
        width: 50vw;
        margin: auto; 
    }