Author of crass here. That's interesting: if you have a specific case where the browser doesn't do what you'd expect, I'd love to see it in a Github issue!
I was thinking of the fact that transparent is supposedly an alias for rgba(0,0,0,0), but this doesn't seem to cause an issue when I try it out, in Firefox at least.
https://github.com/mattbasta/crass/issues/new