Simple responsive flexbox grid

flexBoxGrid.html

<style type="text/css" media="all">
  body {
    margin: 0;
  }
  .container {
    display: flex;
    flex-direction: column;
    justify-content: space-between;
  }
  .flex-grid {
    display: flex;
    flex-direction: column;
    align-items: stretch;
    height: 100%;
  }

  .col {
    flex: 1;
    padding: 15px; /* Styling */
  }

  footer {
    align-self: stretch;
    padding: 15px; /* Styling */
    background: lightgrey; /* Styling */
    min-height: 70px; /* Styling */
  }

  @media (min-width: 600px) {
    .container {
      height: 100vh;
    }
    .flex-grid {
      flex-direction: row;
    }
  }
</style>

<div class="container">
  <div class="flex-grid">
    <div class="col"></div>
    <div class="col" style="background: lightblue;"></div>
    <div class="col"></div>
  </div>
  <footer></footer>
</div>