Steps to reproduce:

1. Create a custom tag like shown in the documentation - https://doc.ibexa.co/en/3.3/extending/extending_online_editor/#configure-custom-tags
2. Publish an Article with an added "factbox" tag in the "Intro" field.
3. Remove style attribute from factbox configuration.
4. Edit the Article from step 2.
5. Observe a web browser's console.

Result:
One is presented with the following exception:

Validation of XML content failed: Unknown attribute 'style' of RichText Custom Tag 'factbox'

Additional error is popping when clicking inside of a factbox:

Uncaught TypeError: Cannot read properties of null (reading 'includes')

Expected result:
A missing attribute should be handled gracefully, without throwing exceptions.

Note:
Seems like the issue doesn't occur on v3.3. Perhaps this is because one cannot enter any content inside the factbox.