What is the stable9 theme? When should I use it instead of stable?

Question

https://www.drupal.org/node/3066038

Has this cryptic statement:

However, defaulting to base theme: stable prevents automatic updates to the next major version of Drupal, since each major version may have a different version of "the stable theme"

and proceeds to list these without any explanation.

The READMEs in the relevant directories are no help.

Warning: Themes that opt out of using Stable as a base theme will need continuous maintenance as core changes, so only opt out if you are prepared to keep track of those changes and how they affect your theme.

Warning: Themes that decide to not use Stable 9 as a base theme will need continuous maintenance as core changes, so only opt out if you are prepared to keep track of those changes and how they affect your theme.

I have a Drupal 8 theme, I am upgrading to Drupal 9 should I specify stable or stable9? This is really confusing.

Answer 1

My theme is extending Drupal 8 Stable, what should I do?

Upgrading to a Stable 9 can be a tedious task. Therefore, we have decided to continue to support previous versions of Stable. Drupal 8 Stable will be shipped in Drupal 9 along Stable 9. Drupal 8 Stable will be deprecated during the Drupal 9 lifecycle to be removed in Drupal 10 so that the theme can be moved into a contrib project.

https://www.drupal.org/node/3107179

Answer 2

If you want to use it you have to enable the theme before using it.

e.g. drush theme:enable stable9

From the .info

description: A base theme using Drupal 9.0.0's core markup and CSS.

So in other words all the new markup in html plus the old markup, css etc in d9+