brk/website_next/learn/style.css

main.learn {
  --sticky-h: 5rem;
  --sidebar-top: 6rem;
  --sidebar-bottom: 1rem;

  display: flex;
  overflow: hidden;

  article {
    position: relative;
    counter-reset: theme;
    min-width: 0;
    min-height: 0;
    overflow: auto;
    scroll-behavior: smooth;
    scrollbar-gutter: stable;

    &::before {
      content: "";
      position: sticky;
      top: 0;
      z-index: 2;
      display: block;
      width: min(100%, 52rem);
      height: var(--sticky-h);
      margin-inline: auto;
      margin-bottom: calc(-1 * var(--sticky-h));
      background: var(--black);
      pointer-events: none;
    }

    > section {
      counter-increment: theme;
      counter-reset: topic;
      width: min(100%, 52rem);
      margin-inline: auto;
    }

    > section + section {
      margin-top: 8rem;
    }

    section section {
      counter-increment: topic;
      margin-top: 4rem;
    }
  }

  h1,
  h2 {
    position: sticky;
    background: var(--black);
    line-height: 1;

    a {
      position: relative;
      display: inline-block;
      color: white;
      text-decoration: none;

      &::before {
        position: absolute;
        top: 50%;
        right: 100%;
        translate: 0 -50%;
        opacity: 0;
        user-select: none;
        text-decoration: none;
      }

      &:hover::before {
        opacity: 0.5;
      }

      &:hover {
        text-decoration: underline;
        text-decoration-thickness: 1px;
        text-underline-offset: 0.125em;
      }

      &:active {
        color: var(--orange);
      }
    }
  }

  h1 {
    z-index: 3;
    top: var(--sticky-h);
    scroll-margin-top: var(--sticky-h);
    padding-bottom: 0.5rem;
    border-bottom: 1px solid var(--dark-gray);
    font-size: 2.75rem;

    a::before {
      content: counter(theme, upper-roman) ". ";
    }
  }

  h2 {
    z-index: 1;
    top: var(--sticky-h);
    scroll-margin-top: var(--sticky-h);
    padding-top: 3.5rem;
    padding-bottom: 0.5rem;
    border-bottom: 1px dashed var(--dark-gray);
    font-size: 1.5rem;

    a::before {
      content: counter(topic) ". ";
    }
  }

  p {
    margin-top: 1rem;
    color: var(--dark-white);
  }

  figure {
    margin-top: 2rem;
    color: var(--gray);
    font-size: var(--font-size-xs);
    text-transform: uppercase;

    > div {
      height: 18rem;
      display: grid;
      place-items: center;
      border: 1px solid var(--dark-gray);
      font-size: var(--font-size-sm);
    }

    figcaption {
      margin-top: 0.75rem;
    }
  }

  > nav {
    counter-reset: content-theme;
    min-height: 0;
    overflow: auto;
    scrollbar-gutter: stable;
    padding-top: var(--sidebar-top);
    padding-bottom: var(--sidebar-bottom);
    font-size: var(--font-size-xs);
    line-height: var(--line-height-sm);
    text-transform: uppercase;

    ol {
      list-style: none;
      margin: 0;
      padding: 0;
    }

    > ol > li {
      counter-increment: content-theme;
      counter-reset: content-topic;
    }

    ol ol {
      margin-top: 0.5rem;
      margin-left: 1rem;
      color: var(--gray);
    }

    ol ol > li {
      counter-increment: content-topic;
    }

    li + li {
      margin-top: 0.5rem;
    }

    a {
      color: inherit;
      text-decoration: none;

      &::before {
        opacity: 0.5;
      }

      &:hover {
        color: var(--orange);
      }
    }

    > ol > li > a::before {
      content: counter(content-theme, upper-roman) ". ";
    }

    ol ol > li > a::before {
      content: counter(content-topic) ". ";
    }
  }
}

@media (prefers-reduced-motion: reduce) {
  main.learn article {
    scroll-behavior: auto;
  }
}