Adding accordion to CKEditor toolbar gets error

Problem/Motivation

If I add the accordion to the CKEditor toolbar, I get a configuration error.

Steps to reproduce

1. Go to simplytest.me
2. Choose Drupal core as the project (I'm offered 10.3.2 and I accept it)
3. Expand Advanced
4. Add project CKEditor Accordion
5. Click Create
6. Log in
7. Manage > Configuration > Text Formats and Editors
8. On the row for Basic HTML, click Configure
9. Drag the accordion icon into the Active toolbar

Expected result: Drupal accepts my action
Actual result: Drupal gives the error 'The following tag(s) are already supported by enabled plugins and should not be added to the Source Editing "Manually editable HTML tags" field: accordion (<dl>).'

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes