In the ValueFromStringHelper for Servo that is added in bug 1355348, we should factor out the text zoom from font-size values like we do for Gecko style. e.g. computedValue.mServo = Servo_DivideTextZoom(*computedValue.mServo, aPresContext->EffectiveTextZoom()); We might end up tying this to bug 1357295 if we produce a special method for parsing CSS values for SMIL in Servo.
How not? What exactly isn't working?
(Or wasn't working when comment 1 was written, I don't think anything related to what was or wasn't working has significantly changed since then)