I figured out pretty quickly that there is no comprehensive way to test markdown on Lemmy because of differing standards, and because of an infinite combination of different tags, like images inside of spoilers, as you say. I tried to stick to the most common uses I see and which have been referenced and used widely for a long time (which is very subjective but I had to start somewhere). In theory, every combination should work, but we (Lemmy as a whole) don't even have the basics consistent.