Directed percolation (DP) is an archetypical model of phase transitions into an absorbing state, i.e. a state from which a system can never escape. Although the corresponding universality class is well established both in theory and in simulations with plenty of examples, such as in epidemics, forest fires, catalytic reactions, interface grouwth, synchronization and calcium dynamics in living cells, surprisingly, no experiments could show convincing evidence of the DP critical behavior, which has been recognized as an outstanding open problem [1]. In this study, we investigate a transition to spatiotemporal intermittency between two turbulent states (DSM1-DSM2) observed in electrohydrodynamic convection of nematic liquid crystals. Performing two sets of experiments, namely steady-state experiment under constant applied voltages and critical-quench experiment where voltage is suddenly decreased, we observed algebraic scaling laws and measured a complete set of independent critical exponents. Their values all precisely agree with those defining the DP class. Furthermore, data collapse is achieved with the universal scaling function in agreement. This constitutes the first complete and satisfactory experimental realization of a DP class transition [2]. Furthuermore, I show a novel aspect of the DP universality, namely universal scaling of hysteresis loops. The DSM1-DSM2 transition is accompanied by hysteresis, whose width algebraically depends on the ramp rate of the applied voltage [3]. This scaling has been unexplained since it was found, but in fact, it is an outcome of the DP universality under the presence of a very rare spontaneous nucleation [2]. We obtained the power of the scaling which quantitatively agrees with experiments, not only in numerical simulations but also by a simple theoretical calculation, where the power of the scaling turns out to be simply related to a critical exponent of DP. These results show that the scaling of the hysteresis loops is universal among transitions into a quasi-absorbing state, suggesting that it can be observed in a variety of other systems. [1] H. Hinrichsen, Adv. Phys. 49, 815 (2000). [2] K. Takeuchi, M. Kuroda, H. Chate and M. Sano, to be published. [3] S. Kai, W. Zimmermann, M. Andoh, and N. Chizumi, Phys. Rev. Lett. 64, 1111 (1990).