*,
  *::after,
  *::before {
    box-sizing: border-box;
  }

  :root {
    font-family: Arial, Helvetica, sans-serif;
  }

  .container {
    max-width: 80rem;
    margin: 0 0;
    padding: 0 0.5em;
  }

  body {
    padding: 0 0 1em;
  }

  header {
    background-color: #333;
    color: #fff;
    border: 5px solid #000;
  }

  header ul {
    list-style: none;
    margin: 0;
    padding: 0;
  }

  header a {
    color: #fff;
    text-decoration: none;
    display: block;
    padding: 0.5em 1em;
    border-top: 1px solid #999;
  }

  header .title {
    font-size: 150%;
    font-style: italic;
    font-weight: bold;
    padding: 1em;
  }

  main {
    padding: 0 1em;
  }

  .cards {
    list-style: none;
    margin: 0;
    padding: 0;
  }

  .cards li {
    border: 5px solid #000;
    margin-bottom: 1em;
  }

  .cards h2 {
    background-color: #333;
    color: #fff;
    margin: 0;
    padding: 0.5em 1em;
  }

  .cards .inner {
    padding: 0.5em 1em;
  }

  .sidebar {
    background-color: #333;
    border: 5px solid #000;
    padding: 0.5em 1em;
    color: #fff;
  }

  /* --- Responsive layout: from 40em (≈640px) --- */
  @media screen and (min-width: 40em) {
    .container {
      display: flex;
      max-width: 80rem;
      /* stop scaling beyond this width */
      margin: 0 auto;
    }

    header {
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 0;

      ul {
        display: flex;
        align-items: center;
      }

      a {
        border-top: none;
      }
    }

    .cards {
      display: grid;
      grid-template-columns: repeat(auto-fill, minmax(15em, 1fr));
      gap: 1em;
    }

    .sidebar {
      margin: 1em;
    }
  }