Hi Dmytro,
Welcome to the community.
As you've seen this isn't possible at the moment, but we are tracking a similar feature request for select fields and I suggested a workaround suggested using regular expressions there:
You would need to combine something like that with the HTML Serializer since this is in the Rich Text field:
I'm tracking this as a separate feature request also as it's slightly different than the original request. Though I don't think the product team will implement this in the near future as they are planning an overhaul of the Rich Text field and this would have to be considered alongside that work.
I hope this helps,
Thanks.