Given the commits, there hasn't been much activity on HSS after 2013.
https://github.com/ncannasse/hss
I would also suggest to use LESS or SASS since both projects are very active and maintained.
As said, I think CSS is not a programming language but markup styling, so you could wonder how CSS could be a possible target.
I could imaging one creates a fancy lib that parses and writes Haxe objects to stylesheets and back, but maybe something like that is already done..?
http://lib.haxe.org/search/?v=css
Greetings,