How can I create a responsive grid layout using only HTML and CSS?

<!-- BEGIN COPY / PASTE -->
    <style>
      .grid-container {
        display: grid;
        grid-template-columns: repeat(auto-fill, minmax(100px, 1fr));
        gap: 10px;
      }
      .grid-item {
        background-color: #f0f0f0;
        padding: 20px;
        text-align: center;
      }
    </style>

    <div class="grid-container">
      <div class="grid-item">1</div>
      <div class="grid-item">2</div>
      <div class="grid-item">3</div>
      <div class="grid-item">4</div>
      <div class="grid-item">5</div>
      <div class="grid-item">6</div>
    </div>
    <!-- END COPY / PASTE -->