Nvu had a strong limitation, because of Gecko. It could only edit CSS styles understandable by the embedded version of Gecko. This is something I don't want for BlueGriffon because I think it does not make sense to make a content editor for the Web that is restricted to Gecko-based browsers. I want users to be able to manipulate a UI that will create -moz-transform but also -webkit-transform. And I also want users to be able to edit stylesheets that include both properties while the -webkit-* are never present in Gecko's CSS OM (and that's normal). I don't need to cascade, I want to preserve all rules and all declarations even multiple declarations of the same property, I want to preserve comments as much as possible (ie between rules and between declarations), I want to preserve CSS parsing errors. The only extra thing I need is a resolver for shorthands.
So I just started my own CSS parser. It'll parse a string containing a stylesheet and will return CSSOM-like objects with the necessary extensions. It'll live inside the CSS Inspector sidebar of BlueGriffon but I will probably make it MPL.