Hiding sections before we choose to show them

  <style>
    #SectionA,
    #SectionB {
      display: none;
    }
  </style>

Running code once page has loaded

  <script>
    document.addEventListener("DOMContentLoaded", function(event) {


    });
  </script>

Running code on page load

  <script>
    window.onload = function()
    {

    };
  </script>