code: function labeledByATab(element) {
          if (element.hasAttribute('aria-labelledby')) {
              var labelingElements = document.querySelectorAll('#' + element.getAttribute('aria-labelledby'));
              return labelingElements.length === 1 && labelingElements[0].getAttribute('role') === 'tab';
          }
          return false;
      }
docs: Checks if the tabpanel is labeled by a tab
  
  @param {Element} element the tabpanel element
  @returns {boolean} the tabpanel has an aria-labelledby with the id of a tab