Section Styles: Fix removal of insecure properties for block style variations

[aaronrobertshaw]

This PR backports the PHP changes from WordPress/gutenberg#66896

These changes prevent valid inner block type and element styles from being stripped out of block style variations via remove_insecure_properties.

Trac ticket: https://core.trac.wordpress.org/ticket/62372

Testing Instructions

1. Ensure unit tests are passing
2. Using a multisite install, login with a user that has access to the Site Editor but doesn't have unfiltered_html caps e.g. site admin
3. Activate a theme that within a theme style variation defines a block style variation containing inner block and element styles
4. Navigate to Appearance > Editor
5. Edit a page or template and apply the block style variation from step 3
6. Save changes
7. Apply the theme style variation identified in step 3.
8. Re-save changes and switch to the frontend
9. Confirm that the block with the block style variation applied gets the correct styles defined within the theme style variation.

More detailed instructions testing with the Assembler block theme can be found on the original Gutenberg PR. It also has some test block markup.

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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.

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Props aaronrobertshaw, peterwilsoncc, mukesh27, andrewserong.

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[andrewserong]

This is testing nicely for me. I followed similar testing instructions to WordPress/gutenberg#66896 using the Assembler theme to test the output of section styles, and via applying the Noir styles variation. With this PR applied, (and with color-mix enabled in kses.php), the section styles rules are output as expected:

As in the Gutenberg PR, I like how this splits the logic out into separate methods, and that the tests are explicit about which things should be stripped — both help improve readability to me 👍

LGTM! 🚀

[peterwilsoncc]

This looks good to me and I've validated the fix by comparing the tests on this branach and on trunk.

I see Andrew's done some manual testing too.

[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 59802
GitHub commit: d71f29f

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.