Hide a section of the page

Use .style.display = just make sure when you show it you are using ‘flex’ and not ‘inline’ if the element is a flex!

Hide it:
  document.getElementById('MyId').style.display = 'none';
Show it
  document.getElementById('MyId').style.display = 'inline';
//or
  document.getElementById('MyId').style.display = 'flex';

To hide a section with id=pageheader”

  <script>
    //Hide section when the page loads
    document.addEventListener("DOMContentLoaded", function() {
        var header = document.getElementById("page-header");
        if (header) {
            header.style.display = "none";
        }
    });
  </script>
Display it again
  header.style.display = 'inline';