WordPress: Split block styles into maybe enqueues for classic themes.

block-style-splitter.php

<?php

/**
 *
 * @link              https://www.spacedmonkey.co.uk
 * @since             1.0.0
 * @package           Style_Splitter
 *
 * @wordpress-plugin
 * Plugin Name:       Block style splitter
 * Plugin URI:        https://www.spacedmonkey.co.uk
 * Description:       Split block styles into maybe enqueues for classic themes. 
 * Version:           1.0.0
 * Author:            Jonny Harris
 * Author URI:        https://www.spacedmonkey.co.uk
 * License:           GPL-2.0+
 * License URI:       http://www.gnu.org/licenses/gpl-2.0.txt
 * Text Domain:       style-splitter
 * Domain Path:       /languages
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

add_filter( 'should_load_separate_core_block_assets', '__return_true' );


/**
 * Get all post in main loop and sidebar widgets and get there styles.
 *
 */
function maybe_inline_all_styles(){
	global $wp_query, $wp_registered_widgets, $wp_registered_sidebars;

	foreach( $wp_query->get_posts() as $post ){
		render_contet_block_styles( $post->post_content );
	}

	$sidebars_widgets = get_option( 'sidebars_widgets' );

	foreach ( (array) $wp_registered_sidebars as $index => $sidebar ) {
		if ( ! empty( $sidebars_widgets[ $index ] ) ) {
			foreach ( (array) $sidebars_widgets[ $index ] as $widget ) {
				if ( array_key_exists( $widget, $wp_registered_widgets ) ) {
					if ( !array_key_exists( 'callback', $wp_registered_widgets ) ) {
						continue;
					}
					foreach( $wp_registered_widgets[$widget]['callback'] as $callback ) {
						if (!$callback instanceof WP_Widget_Block) {
							continue;
						}
						foreach ($callback->get_settings() as $setting) {
							if (isset($setting['content'])) {
								render_contet_block_styles($setting['content']);
							}
						}
					}
				}
			}
		}
	}
}

add_action( 'template_redirect', 'maybe_inline_all_styles' );

/**
 * Parse content and convert into blocks and enqueue styles. 
 *
 * @param string $content Content with blocks.
 */
function render_contet_block_styles( $content ){
	global $post;
	$blocks = parse_blocks( $content );
	foreach( $blocks as $parsed_block ){
		$parsed_block = apply_filters( 'render_block_data', $parsed_block, $parsed_block, null );

		$context = array();

		if ( $post instanceof WP_Post ) {
			$context['postId'] = $post->ID;
			$context['postType'] = $post->post_type;
		}

		$context = apply_filters( 'render_block_context', $context, $parsed_block, null );

		$block = new WP_Block( $parsed_block, $context );

		if ( ( ! empty( $block->block_type->script_handles ) ) ) {
			foreach ( $block->block_type->script_handles as $script_handle ) {
				wp_enqueue_script( $script_handle );
			}
		}

		if ( ! empty( $block->block_type->view_script_handles ) ) {
			foreach ( $block->block_type->view_script_handles as $view_script_handle ) {
				wp_enqueue_script( $view_script_handle );
			}
		}

		if ( ( ! empty( $block->block_type->style_handles ) ) ) {
			foreach ( $block->block_type->style_handles as $style_handle ) {
				wp_enqueue_style( $style_handle );
			}
		}
	}
}

Heyo! Got here from Twitter while doing my own testing of per-block styles only on pages where the block is loaded... what's the TL;DR of the benefits of your approach here? If you don't mind sharing ;)