PWA extension plugin to facilitate precaching styles, as an alternative to using WP_Service_Worker_Styles_Integration. See https://wordpress.org/support/topic/precaching-best-practices/

pwa-precache-styles.php

<?php
/**
 * Plugin Name: PWA Precache Styles
 *
 * @package   PWA_Precache_Styles
 * @author    Weston Ruter, Google
 * @license   GPL-2.0-or-later
 * @copyright 2019 Google Inc.
 *
 * @wordpress-plugin
 * Plugin Name: PWA Precache Styles
 * Description: Add styles with the 'precache' data to the precache so they will be served cache-first and be available while offline. Depends on the <a href="https://wordpress.org/plugins/pwa/">PWA plugin</a>. To flag a given stylesheet for precaching, just do <code>wp_style_add_data( $handle, 'precache', true )</code> after registering it (e.g. at the <code>wp_enqueue_scripts</code> action).
 * Plugin URI:  https://gist.github.com/westonruter/caa6e08b8d1f70cd3ddab1064d7d1bc2
 * Version:     0.1.0
 * Author:      Weston Ruter, Google
 * Author URI:  https://weston.ruter.net/
 * License:     GNU General Public License v2 (or later)
 * License URI: http://www.gnu.org/licenses/gpl-2.0.html
 */

namespace PWA_Precache_Styles;

/**
 * Register each style with 'precache' data for precaching.
 *
 * @eee WP_Service_Worker_Styles_Integration
 * @see \WP_Styles::_css_href()
 * @see \WP_Styles::do_item()
 * @link https://github.com/xwp/pwa-wp/blob/0.3.0/integrations/class-wp-service-worker-styles-integration.php
 * @param \WP_Service_Worker_Scripts $scripts Scripts.
 */
function register_styles_for_precaching( $scripts ) {
	if ( ! $scripts instanceof \WP_Service_Worker_Scripts ) {
		_doing_it_wrong( __FUNCTION__, 'Argument is not instance of WP_Service_Worker_Scripts. Perhaps PWA plugin changed?', '0.1' );
		return;
	}
	if ( ! method_exists( $scripts->precaching_routes(), 'register' ) ) {
		_doing_it_wrong( __FUNCTION__, 'Unable to locate WP_Service_Worker_Scripts::precaching_routes()->register() method. Perhaps PWA plugin changed?', '0.1' );
		return;
	}

	foreach ( wp_styles()->registered as $handle => $dependency ) {
		if ( empty( $dependency->extra['precache'] ) ) {
			continue;
		}

		// This code is copied from \WP_Styles::_css_href() and \WP_Styles::do_item().
		$url = $dependency->src;
		if ( is_string( $url ) && ! preg_match( '|^(https?:)?//|', $url ) && ! ( wp_styles()->content_url && 0 === strpos( $url, wp_styles()->content_url ) ) ) {
			$url = wp_styles()->base_url . $url;
		}

		// This code is copied from .
		if ( null === $dependency->ver ) {
			$ver = '';
		} else {
			$ver = $dependency->ver ? $dependency->ver : get_bloginfo( 'version' );
		}

		// Amend the version to the URL.
		if ( ! empty( $ver ) ) {
			$url = add_query_arg( compact( 'ver' ), $url );
		}

		/** This filter is documented in wp-includes/class.wp-styles.php */
		$url = apply_filters( 'style_loader_src', $url, $handle );

		$scripts->precaching_routes()->register( $url, $ver );
	}
}

add_action( 'wp_front_service_worker', __NAMESPACE__ . '\register_styles_for_precaching' );