Or maybe remove the banner option from the compass task and add `chosen.css` to the concat task (after compass of course)

```coffee
concat:
  options:
    banner: "<%= comments %>"
  jquery:
    src: ["public/chosen.jquery.js"]
    dest: "public/chosen.jquery.js"
  proto:
    src: ["public/chosen.proto.js"]
    dest: "public/chosen.proto.js"
  css:
    src: ["public/chosen.css"]
    dest: "public/chosen.css"
```