nav {
    display: flex;
    justify-content: space-around;
    position: sticky;
    top: 0;
}
nav > a {
    flex: 1;
    text-align: center;
}
nav > a:hover { font-weight: bold }
#prev { background-color: #a40000 }
#up { background-color: #4e9a06 }
#next { background-color: #204a87 }

img {
    display: block;
    margin: auto;
    max-width: 100%;
}