This crate implements CSS parsing and Flexbox layout. CSS parsing relies on the CSS Parser from Servo. Flexbox is implemented with Stretch, albeit currently a fork by msilgreith, cloned into here to serve a few small changes (a change for more thread safety, and to push appearance based styles that Flexbox doesn't concern itself with). Down the road, I could see this not including Stretch inline.
Open an issue, or hit me up on Twitter.