html {
    background-color: greenyellow;
}

header {
    text-align: center;
}

header h1{
    display: inline;
    border-bottom: 3px;
    border-color: red;
}

nav {text-align: center;}

nav ul {
    list-style-type: none;
    overflow: hidden;
}

nav ul > li {
    display: inline-block;
    margin-inline: 12px;
}

main,footer {
    margin: 0 auto;
    max-width: 800px;
}