Add custom CSS support per block and in Global Styles

[ntsekouras]

Adding custom CSS was available to users in customizer in classic themes, but there is no option to add arbitrary CSS via the site editor.

The ability to add custom CSS was introduced here: ​https://github.com/WordPress/gutenberg/issues/30142

[ntsekouras]

I'd like to propose the custom CSS support per block and in Global Styles to be a blessed task in the WordPress 6.2 release cycle.

When was this first released?
This feature was introduced in Gutenberg 14.8: ​https://github.com/WordPress/gutenberg/pull/46141

Why blessed it?
This new piece of UI might require UX changes to address accessibility needs or to polish some interactions. As an example you can see some remaining tasks in this issue: ​https://github.com/WordPress/gutenberg/issues/30142

[hellofromTonya]

There is a discussion underway in ​in Core Slack core-committers channel for code being copied/merged from Gutenberg into Core already being blessed.

Today is 6.2 Beta 1. Core's handbook is clear about how to handle ALL enhancements and features requests before starting Beta 1.

• ​How the Release Cycle Works:

  Phase 3: Beta. Betas are released and beta-testers are asked to start reporting bugs. No more commits for new enhancements or feature requests are allowed for the rest of the release.
  

• ​Releasing Beta and RC Versions:

  For Beta releases, ensure that the milestone is cleared of enhancement and feature request tickets (e.g., Trac query for 6.1). If there is agreement within the release squad that a certain enhancement or feature request is critical for the release and thus needs to remain in the milestone, its ticket type should be changed to “Task (blessed)”. Also ensure with the Editor Tech Lead that enhancements/features merges from Gutenberg are similarly either cleared from the milestone or changed to a “Task (blessed)”.
  

This discussion likely will not get resolved before today's Beta 1 party starts.

I'm setting this new feature to "blessed" as it is planned for 6.2 and has already been released in Gutenberg. Its continuation is feature refinement. Given the fast iterations and release cycles, it will receive more testing, discussion, and feedback through Gutenberg's cycles than in Core's beta cycle.

This "blessed" feature can be revisited once there's resolution to the discussion.

In talking with @priethor, there will be a mixture of PHP code to merge/backport and packages to update. This ticket can be the landing place to track the feature.

[hellofromTonya]

The individual commits made for this enhancement need to be copied here from the @wordpress package updates #57471. That'll wire the changes to the feature.

As RC1 is happening in a few hours, closing this ticket.