RTF editor may be part of WYSIWYG solution but it also might be used to edit text that on publication will be looking entirely differently or will be used in various places.

Ohh, I think I get it now – is this about literally using RTF as an intermediate format? I really don’t think I’ve heard about it being used in the context of static sites, but I see how it might make sense as a way of storing, well, rich text.

Pandoc is your best friend in such cases: https://pandoc.org/