Source

File: wp-includes/functions.wp-styles.php

function wp_print_styles( $handles = false ) {
	global $wp_styles;

	if ( '' === $handles ) { // For 'wp_head'.
		$handles = false;
	}

	if ( ! $handles ) {
		/**
		 * Fires before styles in the $handles queue are printed.
		 *
		 * @since 2.6.0
		 */
		do_action( 'wp_print_styles' );
	}

	_wp_scripts_maybe_doing_it_wrong( __FUNCTION__ );

	if ( ! ( $wp_styles instanceof WP_Styles ) ) {
		if ( ! $handles ) {
			return array(); // No need to instantiate if nothing is there.
		}
	}

	return wp_styles()->do_items( $handles );
}