An underrated accessibility feature on Lemmy is the ability to add alternative text to images in Markdown. The image above will be read as "photo of a snail" by screen-readers. The Markdown for that one is:
As you can see, the alternative text simply needs to be inserted between the two brackets, which are empty by default when you insert an image.
Markdown is not Lemmy tho, it's a markup language independent of the platform, like HTML, reddit uses it as well, also github and mediawiki (wikipedia) and many others.
While each platform can provide formatting help, it's not the entire language, have a look here: https://www.markdownguide.org/
How the markdown language is implemented is highly platform-dependent though. For instance, in Reddit you cannot insert an image in Markdown-mode (and you don't even have alt-text even when using their "fancy pants" mode).
True, but the language is independent from that.
I mean, the language itself has a set of "features", not all of them are implemented on all platforms as you rightfully said, but it's not that the language changes because there are differences in the implementations.