Spoilers, subscripts and superscripts don’t work, but everything else does.
Markdown in general is unfortunately rather difficult to render. We use an open-source treesitter that parses GitHub-flavored markdown, which is similar to Lemmy’s markdown. Fully supporting the Lemmy markdown specification (spoilers, for example) would probably require us to instead use a custom implementation built directly out of cmark.
We have a very small developer team working in our free time, and none of us have any prior knowledge in the area of low-level markdown extensions. It’s very complex, and we’d probably never leave beta if making a perfect markdown renderer was on our requirement list, so we omitted it. It might be possible to work out given enough time, but that’s time that we chose to invest in other areas instead.
If you know of any other non-web apps (iOS or Android) that are able to correctly render all of Lemmy’s markdown, let us know. We may be able to learn from/use their implementation, depending on the license it’s distributed under. I haven’t seen any Lemmy clients that can do this yet.