Stylo is the project name for taking Servo's CSS system and hooking it up to Gecko.
We conditionally compile out unsupported stuff. There is some complexity in shared code introduced but very little of it is anything but a complication of the abstractions (not an actual behavior-level change which can have a perf impact).
So for example the weird cascading behavior exists when you compile pure servo (because that's a core functionality of font-size, which servo does implement), but servo doesn't have to deal with the mathml stuff or text-zoom. Servo has some no-op code for text-zoom that lets shared code be written cleaner, but it compiles out.
We conditionally compile out unsupported stuff. There is some complexity in shared code introduced but very little of it is anything but a complication of the abstractions (not an actual behavior-level change which can have a perf impact).
So for example the weird cascading behavior exists when you compile pure servo (because that's a core functionality of font-size, which servo does implement), but servo doesn't have to deal with the mathml stuff or text-zoom. Servo has some no-op code for text-zoom that lets shared code be written cleaner, but it compiles out.