Important: #102507 - Default CKEditor5 allowed classes and data attributes configurated reverted

See forge#102507

Description

With TYPO3 v12.4.7 (see forge#99738) an option to allow all classes in CKEditor5 has been enabled in the TYPO3 default configuration which implicitly caused all custom html elements to be allowed. This rule has now been dropped from the default configuration:

htmlSupport:
  allow:
    - { classes: true, attributes: { pattern: 'data-.+' } }

The configuration matched to any HTML element available in the CKEditor5 General HTML Support (GHS) schema definition. This became an issue, since CKEditor5 relies on the set of allowed elements and classes when processing content that is pasted from Microsoft Office.

Installations that relied on the fact that v12.4.7 allowed all CSS classes in CKEditor5 should encode the set of available style definitions via editor.config.style.definitions which will make them accessible to editors via the style dropdown toolbar element:

style:
  definitions:
     - { name: "Descriptive Label", element: "p", classes: ['my-class'] }

Custom data attributes can be allowed via General HTML Support:

htmlSupport:
  allow:
    - { name: 'div', attributes: ['data-foobar'] }