In the real world, I need to write documentation that can be viewed in our app’s online documentation viewer and can be previewed in GitHub (because writers don’t want to have to decipher broken markdown diffs in the pull request UI). That means any extensions that we’re going to use have to be in GitHub and in the app.
Or just use HTML for those rare cases. I write
<a class=next href=page2.html>next</a>, it gets the next-button styling in our app, but GitHub just strips the class attribute and renders it like a link. It’s because an HTML parser can parse a tag without recognizing it (and, in a sandboxed environment like GitHub, strip anything it doesn’t understand). Markdown syntax can’t do that.
Edit: to be clear, I don’t really care that it’s HTML per se. What really matters is that it’s a way to extend Markdown that doesn’t look like garbage in oblivious viewers. But HTML has the big advantage of already existing, already having tooling, and already being implemented by most major Markdown vendors (except Reddit, so annoying).