TYPO3 Core

The rendering of textmedia is by core only provided for fluid_styled_content.

Yes, but i should also be provided for css_styled_content.

No, Textmedia is a specific element of Fluid Styled Content and has nothing todo with css_styled_content.

css_styled_content should not be used in combination with fluid_styled_content.

I don't understand why textmedia shouldn't be available in css_styled_content.
Yes, I aware of the fact, that they shouldn't be mixed.

But for example the types "bullets", "tables" or "menu" can also be used by css_styled_content_ AND fluid_styled_content.
When you include fluid_styled_content the TS changes from tt_content.menu = COA to tt_content.menu < lib.fluidContent and so on.

For textmedia that can be reproduced with:
tt_content.textmedia < tt_content.textpic
tt_content.textmedia.20.imgList.cObject.references.fieldName = assets

I can understand that all new Typo3 Installations should use fluid_styled_content, but when I update an old one, I won't rewrite my whole code for fluid_styled_content

Than you can still use css_styled_content. No need to use fluid_styled_content here.

Textmedia is a element which are several elements in css_styled_content.

Yes I want to stay with css_styled_content, but I also want to use the "Add Media by URL" feature in the textmedia element.

That is not possible and will not be made possible in the TYPO3 CMS core.
The add media by URL uses a new concept to process its data. This is not available within css_styled_content.