The problem is not "CSS" alone, and a pure CSS editor is of little interest to me. Web page - HTML+CSS - creation is my concern. For it to be worthwhile, it would need to be paired with an equally capable HTML editor, the combined output making a complete web page. Even then, it seems clumsy and clunky compared to a single GUI, and I very strongly suspect it would meet with very little market acceptance.
And frankly,
again, I am not interested that specialists find it easy to use. I am interested in the web in terms of mass media, and most people find CSS+HTML frankly arcane, since there is a distinct and notable lack of any even remotely decent WYSIWYG editors!
And no, unlike say XSL, CSS handles inheritance very badly*. It has little concern for anything except expert hand-editing, it's near-impossible for a consistent set of rules to be applied to allow automagic generation of reasonable script, especially since you need to essentially do over for each of the major layout engines.
As I said, you can't even be sure of the cascade order for many tags.
Which again is the issue, and why we DON'T have a reasonable WYSIWYG editor... (along with the basic issue of making natural layouts HARD!)
(*Not that I entirely agree with how XSL does it either. But my own ideas on handling inheritance both ways are something I've incorporated into the XML-based layout format of my data-driven game-editor design and that's propitiatory. Plus, my ideas are not precisely new, just specialised and as far as I know no game editor does anything like it - certainly none on the market do
)