example.scss

* {
    margin: 0; padding: 0;
}

body {
    font: normal 16px/24px 'Helvetica Neue', Helvetica, Arial, freesans, sans-serif;
}

main {
    width: 960px; margin: 0 auto; overflow: hidden;
}

#contents {
    position: fixed; top: 40px; width: 320px;

    ol {
        counter-reset: section;
        list-style-type: none;
    }

    li:before {
        counter-increment: section;
        content: counters(section, '.') '. ';
    }
    
    > ol {
        background: #FFF8DC; border: 1px solid #E0DCBF; padding: 10px;
    }

    a {
        color: #0000ee; text-decoration: none;

        &:hover {
            text-decoration: underline;
        }
    }

    .active-anchor {
        > a {
            color: #f00;
        }
    }

    .active-child {
        > a {
            &::after { content: ' (active child)'; text-decoration: none; color: #444; }
        }
    }
}

article {
    overflow: hidden; position: relative; margin: 40px 0 40px 340px; background: #fff; border: 1px solid #ccc; padding: 20px 40px 40px 40px;

    h1,
    h2,
    h3,
    h4 {
        font-size: 30px; line-height: 40px; margin: 0 0 10px 0; padding: 0; font-weight: bold;
    }

    h1 {
        margin: 0 0 20px 0; padding-top: 20px;
    }

    h2,
    h3,
    h4 {
        font-size: 20px; line-height: 30px; padding-top: 20px;
    }

    h3,
    h4 { font-size: 16px; line-height: 24px; }

    p {
        margin: 10px 0;
    }
}