I am not sure how is that suppose to work, but with Slurp, I use ace for html [0] and gcss for css[1].

You can find their bindings here: https://github.com/slurp-contrib

I haven't got the time to put these in the docs just yet.

[0]: https://github.com/yosssi/gcss

[1]: https://github.com/yosssi/ace