Which give even more credence to what the parent said: that "CSS isn't the right abstraction for styles in a text editor…".

We don't really need the "more power" in an editor, but we could use the less slugginess (and less conceptual overhead) than what CSS offer.

>Is it worth restricting an editor to only styles that don't influence block layout? For example, should it be impossible for a theme to make comment lines taller or shorter?

I'm pretty sure existing editors can do that today too, without CSS. Setting a different font size for comment lines for example.