How to stack a portable countertop / tabletop Dishwasher on top of a front load washing machine?
This can definitely be done, although the way I did it was to create a countertop over the washing machine, boxing in the washing machine, (the same as any countertop you would normally sit the tabletop dishwasher) thereby making them separate entities.
All that needed to be done was to drill a fair sized hole for the pipes and cables to fit through the countertop behind the dishwasher, not visible, and the dishwasher can come forward to be flush with the countertop. It is never going to topple over, why would it, it's only the door that will venture out  and that won't topple it. (I have a Bosch countertop dishwasher, and it has never ever unbalanced) I'm even considering boxing in the dishwasher so that the microwave can sit on top. (I don't have any cabinets above, to get in the way.) and I can reach it quite easily, like an eye level oven.
The only thing that may be a safety issue is if all appliances are sharing one outlet. I would make sure they each had their own  so as not to overload it. Overall, it has worked for me and it's all next to the kitchen sink so the plumbing is all in one place, nice and tidy.