我问了一个更一般的问题（非特定于WordPress）question about polyfilling on StackOverflow （我强烈建议通读随后的评论以了解上下文），这让我得到了以下答案：

add_action( \'login_enqueue_scripts\', \'wpse_215432_oldbrowsersupport\');
function wpse_215432_oldbrowsersupport() {

    wp_register_script( \'ie_html5shiv\', get_stylesheet_directory_uri().\'/js/html5shiv.min.js\', __FILE__, false, \'3.7.3\' );
    wp_enqueue_script( \'ie_html5shiv\');
    wp_script_add_data( \'ie_html5shiv\', \'conditional\', \'lt IE 9\' );

    //Note: Placeholder script (https://github.com/mathiasbynens/jquery-placeholder) covered by a plugin https://wordpress.org/plugins/html5-placeholder-polyfill/

    wp_register_script( \'ie_selectivizr\', get_stylesheet_directory_uri().\'/js/selectivizr.js\', __FILE__, false, \'1.0.3\' );
    wp_enqueue_script( \'ie_selectivizr\');
    wp_script_add_data( \'ie_selectivizr\', \'conditional\', \'lt IE 9\' );

}