nav h2 {
      margin-top: 0;
      font-size: 1.2em;
      color: #ffd700;
    }
    nav details {
      margin-top: 1em;
    }
    nav summary {
      cursor: pointer;
      font-weight: bold;
      color: #ccc;
    }
    nav a {
      display: block;
      padding: 0.4em 0;
      color: #aaa;
      text-decoration: none;
      margin-left: 1em;
    }
    nav a:hover {
      color: #fff;
    }
/* Base (mobile-first) */
body {
  margin: 0;
  font-family: sans-serif;
  background: #1e1e1e;
  color: #f0f0f0;
  flex-direction: column; /* fallback, ignored unless display:flex */
}
nav {
  width: 100%;
  background: #2a2a2a;
  padding: 1em;
  box-sizing: border-box;
}
main {
  margin: 0;
  padding: 1em;
}

/* Desktop layout */
@media (min-width: 601px) {
  body {
    display: flex;
    flex-direction: row;
    min-height: 100vh;
  }
  nav {
    width: 220px;
    height: 100vh;
    position: fixed;
    top: 0;
    left: 0;
  }
  main {
    margin-left: 220px;
    padding: 2em;
    flex: 1;
  }
}