Could Flex build a Dreamweaver copy cat app?

I have this weird idea that I want to make a sudo wysiwyg html tool.
But just for css in the web page itself.

Lets start over.

I want to use a programing lang and a UI kit to make a wys… tool to build web sites much like Dreamweaver. But My tool only focuses on Visual CSS, aka(margin, padding, box model).

Now, to start of course I just need to make an app that draws boxes on the page and holds text.

And thats where I start. I need to make sure that Flex or whatever Flash tool could possibly support this kind of structure. Or am I better off going for tool kits like C and GTK or Cocoa. I know Cocoa can do it. But I would rather spend the time in an app lang that can port elsewhere if need be.