The details/details-for and error-message/error-for relation types were created for ARIA. At the time, no native widget toolkit was using them, so we limited support to Orca's web script.
Moving these out of the web script and documenting their purpose will make it possible for app developers to use them instead of being limited to description, help text, and announcements.
This is the downstream issue for https://gitlab.gnome.org/GNOME/orca/-/issues/514