Further preserve original CSS cascade when hoisting late-printed styles

[westonruter]

Trac ticket: https://core.trac.wordpress.org/ticket/64354

TODO

• Add test for when classic-theme-styles is not enqueued.
• Add test for when global-styles is not enqueued.
• Ensure that block styles enqueued at enqueue_block_assets will get printed after classic-theme-styles via enqueue_block_styles_assets().

Note that we do not need to consider what wp_enqueue_registered_block_scripts_and_styles() outputs because it short-circuits if wp_should_load_block_assets_on_demand() is enabled, and wp_hoist_late_printed_styles() will not run when on-demand loading is disabled.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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Props westonruter, peterwilsoncc.

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[westonruter]

Cascade when should_load_separate_core_block_assets is disabled:

1. HEAD > STYLE#wp-img-auto-sizes-contain-inline-css
2. HEAD > STYLE#wp-emoji-styles-inline-css
3. HEAD > LINK#wp-block-library-css
4. HEAD > STYLE#classic-theme-styles-inline-css
5. HEAD > STYLE#global-styles-inline-css
6. HEAD > LINK#twentytwenty-style-css
7. HEAD > STYLE#twentytwenty-style-inline-css
8. HEAD > LINK#twentytwenty-fonts-css
9. HEAD > LINK#twentytwenty-print-style-css
10. HEAD > STYLE#wp-custom-css
11. BODY > STYLE#block-style-variation-styles-inline-css
12. BODY > STYLE#core-block-supports-inline-css
13. BODY > STYLE#core-block-supports-duotone-inline-css

And when should_load_separate_core_block_assets is enabled:

1. HEAD > STYLE#wp-img-auto-sizes-contain-inline-css
2. HEAD > STYLE#wp-emoji-styles-inline-css
3. HEAD > STYLE#wp-block-library-inline-css
4. HEAD > STYLE#wp-block-archives-inline-css
5. HEAD > STYLE#wp-block-button-inline-css
6. HEAD > STYLE#wp-block-categories-inline-css
7. HEAD > STYLE#wp-block-heading-inline-css
8. HEAD > STYLE#wp-block-image-inline-css
9. HEAD > STYLE#wp-block-latest-comments-inline-css
10. HEAD > STYLE#wp-block-latest-posts-inline-css
11. HEAD > STYLE#wp-block-buttons-inline-css
12. HEAD > STYLE#wp-block-search-inline-css
13. HEAD > STYLE#wp-block-group-inline-css
14. HEAD > STYLE#wp-block-paragraph-inline-css
15. HEAD > STYLE#classic-theme-styles-inline-css
16. HEAD > STYLE#global-styles-inline-css
17. HEAD > LINK#twentytwenty-style-css
18. HEAD > STYLE#twentytwenty-style-inline-css
19. HEAD > LINK#twentytwenty-fonts-css
20. HEAD > LINK#twentytwenty-print-style-css
21. HEAD > STYLE#wp-custom-css
22. HEAD > STYLE#block-style-variation-styles-inline-css
23. HEAD > STYLE#core-block-supports-inline-css
24. HEAD > STYLE#core-block-supports-duotone-inline-css

When the block styles are grouped under the related HEAD > LINK#wp-block-library-css entry, the order is:

1. HEAD > STYLE#wp-img-auto-sizes-contain-inline-css
2. HEAD > STYLE#wp-emoji-styles-inline-css
3. HEAD > LINK#wp-block-library-css
4. HEAD > STYLE#classic-theme-styles-inline-css
5. HEAD > STYLE#global-styles-inline-css
6. HEAD > LINK#twentytwenty-style-css
7. HEAD > STYLE#twentytwenty-style-inline-css
8. HEAD > LINK#twentytwenty-fonts-css
9. HEAD > LINK#twentytwenty-print-style-css
10. HEAD > STYLE#wp-custom-css
11. HEAD > STYLE#block-style-variation-styles-inline-css
12. HEAD > STYLE#core-block-supports-inline-css
13. HEAD > STYLE#core-block-supports-duotone-inline-css

The cascade is the same, with the difference being that block-style-variation-styles, core-block-supports, and core-block-supports-duotone are located in HEAD as opposed to BODY. This is good as it prevents FOUC.

[github-actions]

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[peterwilsoncc]

@westonruter When it comes to writing tests, are you able to include some for non-core block variations too.

[westonruter]

@peterwilsoncc you mean like in 2fb1c42?