body {
    font-family: Arial, sans-serif;
    margin: 2rem;
}
iframe {
    margin: 1rem 0;
    border: none;
    max-width: 100%;
    width: 560px;
    height: 315px;
    display: block;
}
nav ol {
    padding-left: 1.2em;
}
nav li {
    margin-bottom: 0.5em;
}
@media (max-width: 600px) {
    body {
        margin: 0.5rem;
    }
    h1 {
        font-size: 1.3em;
    }
    nav ol {
        padding-left: 0.8em;
    }
    iframe {
        width: 100% !important;
        height: 200px !important;
    }
}