Why are you telling this?? 20mV is not at all a too little hysteresis for a zero crossing detector! In order to hit the zero crossing correctly for both polarities, hysteresis should be as small as possible.
there are (at least) 2 ways to skin this cat: the small hysteresis which is noise prone and a large input (zener limited) with a larger hysteresis.

If you feed the comparator, say, 220V with a resistor and a zener, you can get excellent zero crossing detection with very little noise suspectibility.

You can, of course, make something work with a small hystersis if you take extreme care in layout etc. That, however, takes a lot more "care" than the large input/large hysteresis approach.

Erik