With emerging technologies like 6G, many new applications like autonomous systems evolve which have strict demands on the reliability of data communications. In this work, we consider a system with multiple slowly fading channels that constitute a diversity system. We show that the joint distribution of the fading coefficients has a tremendous impact on the outage performance of a communication system. In particular, we investigate the zero-outage capacity (ZOC) and characterize the joint fading distributions with a strictly positive ZOC. Interestingly, the set of joint distributions, that lead to positive ZOCs, is larger than a singleton in general. We also derive expressions for the maximum ZOC with respect to all possible joint distributions for selection combining (SC) as diversity combining technique at the receiver. For maximum ratio combining (MRC), we characterize the maximum ZOC within a finite number of bits. Finally, the results are evaluated explicitly for the special cases of Rayleigh fading and Nakagami-$m$ fading in order to quantify the ZOCs for common fading models.