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.