I have tested Brackets a bit the latest days, using it beside Emacs. It was not as integrated to the browser as I thought, but it is still useful. I have used it to edit CSS files and mixed files. It looks up all the places that it think might apply to an element in HTML. Unfortunately it does not seem to have one of those features I really would like: Parsing CSS media queries. It does not tell me which media query is used where and what the limits are for them. It does not even show the media queries at all in the Quick Edit (which is the feature we are discussing).
I think it is the same for CSS classes.
I guess they just stopped the development before implementing such useful features. It is of course quite a bit more complex to implement that part, both on the parsing level and UI level.
Still, it is useful. And implementing those simple features in Emacs would not be that hard, I think. A crucial part would be to setup the connection between different files. (A little bit of that is in nXhtml.)
And yes, it could be used with other languages too. Perhaps it can be useful for class-based languages, but that requires good parsing.