def _download_rtd_zip(rtd_version=None, **kwargs):
    if not toolkit.family.startswith('qt'):
        raise ValueError('Downloaded documentation not compatible with {} '
                         'UI toolkit browser'.format(toolkit.family))
    if rtd_version is None:
        rtd_version = _find_rtd_version()
    data_path = os.path.dirname(
        _find_pkg_data_path('help.html', package='ginga.doc'))
    index_html = os.path.join(data_path, 'index.html')
    if os.path.isfile(index_html):
        return index_html
    url = ('https://readthedocs.org/projects/ginga/downloads/htmlzip/'
           '{}/'.format(rtd_version))
    local_path = urllib.request.urlretrieve(url, **kwargs)[0]
    with zipfile.ZipFile(local_path, 'r') as zf:
        zf.extractall(data_path)
    subdir = os.path.join(data_path, 'ginga-{}'.format(rtd_version))
    for s in os.listdir(subdir):
        src = os.path.join(subdir, s)
        if os.path.isfile(src):
            shutil.copy(src, data_path)
        else:
            shutil.copytree(src, os.path.join(data_path, s))
    shutil.rmtree(subdir)
    if not os.path.isfile(index_html):
        raise OSError(
            '{} is missing; Ginga doc download failed'.format(index_html))
    return index_html