add_action('init', 'add_widget_order_class');
function add_widget_order_class() {
	global $wp_registered_sidebars, $wp_registered_widgets;
	$sidebars = wp_get_sidebars_widgets();
	if (empty($sidebars))
		return;
	foreach ($sidebars as $sidebar_id => $widgets) {
		if (empty($widgets))
			continue;
		foreach ($widgets as $i => $widget_id) {
			$order = $i + 1;
			$wp_registered_widgets[$widget_id]['classname'] .= ' widget-order-' . $order;
		}
	}
}

http://konstruktors.com/blog/wordpress/3615-add-widget-order-css-class-sidebar/

Here is a simple filter to automatically add a class attribute like widget-order-1 to all widgets within sidebars (from konstruktors.com)

Add the Order of Widgets Within a Sidebar as a CSS Class to All Widgets

wordpress

PHP