  In the quantum anomalous Hall effect, the edge states of a ferromagnetically
doped topological insulator exhibit quantized Hall resistance and
dissipationless transport at zero magnetic field. Up to now, however, the
resistance was experimentally assessed with standard transport measurement
techniques which are difficult to trace to the von-Klitzing constant R$_K$ with
high precision. Here, we present a metrologically comprehensive measurement,
including a full uncertainty budget, of the resistance quantization of V-doped
(Bi,Sb)$_2$Te$_3$ devices without external magnetic field. We established as a
new upper limit for a potential deviation of the quantized anomalous Hall
resistance from RK a value of 0.26 +- 0.22 ppm, the smallest and most precise
value reported to date. This provides another major step towards realization of
the zero-field quantum resistance standard which in combination with Josephson
effect will provide the universal quantum units standard in the future.
