Add option to enable custom elements rendering

[Mgsy]

📝 Provide a description of the new feature

The current approach for custom elements is similar to HTML comments - we preserve them in the model and in the data pipeline, but we don't render them in the editing view, so for example, this markup:

<internal>
   <p>Foo</p>
</internal>

is visible in the data:

in the model:

but not in the editing view:

This feature request is about enabling custom elements rendering and editing. 

Some notes from the previous work on bringing support for custom elements: https://github.com/ckeditor/ckeditor5/issues/11432#issuecomment-1103686979 

Kind of related: #12838 

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[Joshuajrodrigues]

any update on this ? i have the same issue but with the "text " element

[Witoso]

Handling of custom elements can be defined with General HTML support. I'm not sure what you meant by the "text" element.

[Joshuajrodrigues]

hey again @Witoso , so for this we reached out to support and were notified that this feature is not available. Basically we are having all our existing data from our old RTEEditor nested in a text tag.

existing data is enclosed in text

hello world

no display

without text tag

this actually is a big issue as we also use revision history with auto save, so not only is the data not visible but if the user tried to add anything all the data gets replaced

[Joshuajrodrigues]

Handling of custom elements can be defined with General HTML support. I'm not sure what you meant by the "text" element.

We have actually done this too, by default we are allowing all html with the regex exp for name, still we get nothing

[Witoso]

so for this we reached out to support and were notified that this feature is not available.

I would trust support engineers on that. Looks like a complex case.