text: // activates the third tab in the tab list, and sets focus on the tab panel
  testPageDocument.querySelectorAll('[role="tab"]').forEach(t => {
    t.setAttribute('tabindex', '-1');
    t.setAttribute('aria-selected', 'false');
  });
  
  testPageDocument.querySelectorAll('[role="tabpanel"]').forEach(p => {
    p.classList.add('is-hidden');
  });
  
  let thirdTab = testPageDocument.querySelector('#complex');
  let thirdPanel = testPageDocument.querySelector('#complex-complex');
  
  thirdTab.removeAttribute('tabindex');
  thirdTab.setAttribute('aria-selected', 'true');
  thirdPanel.classList.remove('is-hidden');
  thirdPanel.focus();
  