function isValidFocusTarget(el) {
      if (
        el &&
        el !== document &&
        el.nodeName !== 'HTML' &&
        el.nodeName !== 'BODY' &&
        'classList' in el &&
        'contains' in el.classList
      ) {
        return true;
      }
      return false;
    }
Helper function for legacy browsers and iframes which sometimes focus
elements like document, body, and non-interactive SVG.
@param {Element} el