Untitled

<html>
  <head>
    <style>
      body,
      html {
        min-height: 100%;
      }
      .site-container {
        display: flex;
        min-height: 100%;
        flex-direction: column;
      }
      .header {
        position: sticky;
        top: 0;
      }
      .content {
        flex-grow: 1;
      }

      /* not important */
      body,
      html {
        margin: 0;
      }
      .header,
      .content,
      .footer {
        padding: 10px;
      }
      .header {
        background: #eeeeee;
      }

      .content {
        background: #dddddd;
      }

      .footer {
        background: #cccccc;
      }
    </style>
  </head>
  <body>
    <div class="site-container">
      <div class="header">Header</div>
      <div class="content">Content</div>
      <div class="footer">Footer</div>
    </div>
  </body>
</html>