docs: def set_invalid(field, render_kw=None):
      """
      Returns *render_kw* with `invalid` added to *class* on validation errors.
  
      Set (or appends) 'invalid' to the fields CSS class(es), if the *field* got
      any errors. 'invalid' is also set by browsers if they detect errors on a
      field.
  
      """
      if render_kw is None:
          render_kw = {}
      if field.errors:
          classes = render_kw.get('class') or render_kw.pop('class_', '')
          if classes:
              render_kw['class'] = 'invalid {}'.format(classes)
          else:
              render_kw['class'] = 'invalid'
      return render_kw