Default to all hidden

  //----- HIDE ALL OPTIONAL WIDGETS AND SECTIONS OF THE PAGE INITIALLY -----
  $HtmlOutput .= <<<_END
    <style>
      #MyButton1,
      #MyButton2 {
        visibility: hidden;
      }
      #MySection1,
      #MySection2 {
        display: none;
      }
    </style>
_END;

To then display a section

  $HtmlOutput .= <<<_END
    <script>
      document.addEventListener("DOMContentLoaded", function(event)
      {
        document.getElementById('MySection1').style.display = 'flex';
      });
    </script>
_END;

Display it if it exists

<script>
  document.addEventListener("DOMContentLoaded", function(event)
  {
    const el = document.getElementById('MySection1');
    if (el)
      el.style.display = 'flex';
  });
</script>