Can’t give tags rels

function extend_tinymce( $values ) {
    $elements="pre[id|class|title|rel]";
    if ( isset( $values['extended_valid_elements'] ) ) {
        $values['extended_valid_elements'] .= ',' . $elements;
    } else {
        $values['extended_valid_elements'] = $elements;
    }
    return $values;
}
add_filter('tiny_mce_before_init', 'extend_tinymce');

The above snippet is what I use for example, of course you can extend this beyond id, class, title, rel, to things such as height, width, style, name, etc…