Buttons created with Gutenberg enabled creates block validation errors when Gutenberg is deactivated

[carolinan]

Description

Originally reported by @kafleg:
Buttons created with Gutenberg active, causes a block validation error in the editor because the miss match of CSS classes when Gutenberg is deactivated:

Content generated by `save` function:

<div class="wp-block-button"><a class="wp-block-button__link">fdsfsdf</a></div>

Content retrieved from post body:

<div class="wp-block-button"><a class="wp-block-button__link wp-element-button">fdsfsdf</a></div>

Step-by-step reproduction instructions

1. Activate Gutenberg 13.4 or newer, or Gutenberg trunk.
2. In the block editor, create a post or page and add some button blocks.
3. Deactivate Gutenberg.
4. View the post or page you just created and check if there are block validation errors for the buttons.

Screenshots, screen recording, code snippet

No response

Environment info

WordPress 6.0

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[Mamaduka]

Probably related to #40260. The Button block will need a deprecation.

cc @scruffian.

[talldan]

I don't think a block deprecation would be possible though, as it would need to be added to an already released version of WordPress core.

TBH, it's possible to get this issue with any block whenever content is created in a newer block editor and then loaded in an older one.

[t-hamano]

I believe this issue is also related to #41140.
For blocks with __experimentalGetElementClassName, disabling Gutenberg after saving the content will break the block.

This is the list of blocks to which __experimentalGetElementClassName is applied:

• core/audio
• core/button
• core/embed
• core/file
• core/gallery
• core/image
• core/table
• core/video

[draganescu]

It's like @talldan said unlikely to have a solution via a deprecation and also hard to solve: content created with newer editors can become invalid in older editors for editing. Gutenberg guarantees the content itself is independent of Gutenberg and via it's universal format and invisible markup does not depend on WordPress to be consumed. But editing blocks in older versions will be problematic.

I think a cool thing would be if we can detect that the user needs a newer editor and inform them.