The social media clients, such as Amethyst, Primal, Damus, Snort.social, Nostrudel.ninja, etc..
Discussion
Is it related to the kinds and nips supported, or the layout presented to the user?
I ask because theres dozens of clienta that focus on kind 1, but even some of the above offer more general purpose. Take for instance nostrudel which has tools, support for communities, badges and streams.
Markdown (and HTML) is forbidden from kind 1 rendering per NIP01, but could apply for any other kind, or if that dev thinks the NIP is wrong.