The CSSParser that RichFaces uses is not only outdated, but also buggy and unstable. For example v0.9.5 forces CSS2 syntax which doesn't allow '-moz' prefixes, but allows @font-face, OTOH v0.9.7 uses CSS2.1 syntax, which allows for prefixes, but doesn't allow @font-face.
It can be quite painlessly replaced by phloc-css, just by overriding the CompiledCSSResource#getInputStream, but that one can also become outdated.
My proposal is to add a configuration option that would point to a list of CSS processors that need to be applied. This would allow for easy parser swapping, adding minifiers, etc.