How safety capacitor replaces optocoupler
It generally does not replace the function of an optocoupler, it's to control EMI by capacitively coupling the output ground with the input DC ground (after filter and rectifier etc.). 

It should be a "Y" type safety capacitor. Usually it's around 1nF. For example, this one (from a Power Integrations datasheet): 



The optocoupler function (when present) is to isolate the output voltage error (as a current) and send it to the controller which runs from the mains. If there is no optoisolator it may be using a transformer winding as feedback, which can work, but it's not as accurate.