12 column grid mobile first (Interactive Example)

<!doctype html>
<title>Example</title>
<style>
  .grid {
    display: block;
    }
  .box {
    color: white;
    font-size: 4vw;
    padding: 10px;
    background: yellowgreen;
    margin: 10px 0;
    text-align: center;
    }
  @media screen and (min-width: 576px) {
    .grid {
      display: grid;
      grid-template-rows: repeat(6, 1fr);
      grid-template-columns: repeat(12, 1fr);
      grid-gap: 10px;
      }
    .box {
      margin: 0;
      }
    .box:nth-child(1) {
      grid-column: span 12;
      }
    .box:nth-child(2), 
    .box:nth-child(3) {
      grid-column: span 6;
      }
    .box:nth-child(4),
    .box:nth-child(5),
    .box:nth-child(6) {
      grid-column: span 4;
      }
    .box:nth-child(7),
    .box:nth-child(8),
    .box:nth-child(9),
    .box:nth-child(10) {
      grid-column: span 3;
      }
    .box:nth-child(11),
    .box:nth-child(12),
    .box:nth-child(13),
    .box:nth-child(14),
    .box:nth-child(15),
    .box:nth-child(16) {
      grid-column: span 2;
      }
    }
</style>
<main class="grid">
  <div class="box">1</div>
  <div class="box">2</div>
  <div class="box">3</div>
  <div class="box">4</div>
  <div class="box">5</div>
  <div class="box">6</div>
  <div class="box">7</div>
  <div class="box">8</div>
  <div class="box">9</div>
  <div class="box">10</div>
  <div class="box">11</div>
  <div class="box">12</div>
  <div class="box">13</div>
  <div class="box">14</div>
  <div class="box">15</div>
  <div class="box">16</div>
  <div class="box">17</div>
  <div class="box">18</div>
  <div class="box">19</div>
  <div class="box">20</div>
  <div class="box">21</div>
  <div class="box">22</div>
  <div class="box">23</div>
  <div class="box">24</div>
  <div class="box">25</div>
  <div class="box">26</div>
  <div class="box">27</div>
  <div class="box">28</div>
</main>