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

  :root {
    font:1.2em/1.4 Arial, Helvetica, sans-serif;
  }

  body {
    padding: 0;
  }

  header {
    background-color: #333;
    color: #fff;
    border: 5px solid #000;
    width: 100%;
  }

  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;
  }

  @media (min-width: 40em) {
    main{
      display: flex;
      .sidebar{
        margin: 1rem;
      }

      .cards{
        display: grid;
        grid-template-columns: repeat(3,1fr);
        gap: 1rem;
      }
    }
    header{
      display: flex;
      align-items: center;
      nav{
        margin-left: auto;
        display: flex;
      }
      ul{
        display: flex;
        gap: 1rem;
      }
      a{
        border-top: none;
      }
    }
  }