What does “only web app” mean for posteo? Can you not use it with a standard mail app?

You can do remote debugging in general but that requires a computer. I think kiwi uses remote debugging in a way to display the devtools on the same device but I haven’t looked into it more.

This is interesting, thanks! It’s missing some things (for example the network tab pretty much doesn’t work) but it’s the best alternative so far.

Yeah remote debugging works on android too but that requires an external device. I’m looking for something that works on the phone itself.