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Markdown is an attempt to codify formatting conventions that were already long in use for newsgroups and email.Markdown was never really about codifying anything existing. Sure, it took a lot of guidance from existing conventions, but it also invented quite a bit, ignored quite a bit, and compromised quite a bit because of wanting to mostly sit atop HTML.
The most notable deviation from custom is its link syntax, which is simply bad. Where any delimiter was necessary, the longstanding custom was to delimit with angle brackets; and Markdown did allow <https://example.com/>, but its text-with-hyperlink syntax of [text](href) is highly confusing, leads to frequent errors, and makes the huge mistake of using as its delimiters characters that are valid in URLs. (In current spec terms, parentheses are in the set of URL code points <https://url.spec.whatwg.org/#url-code-points>.) This has led to all sorts of trouble. My guess is that angle brackets weren’t used because of the potential for confusion with HTML tags, though there would have been no real parser ambiguity, and autolink syntax kinda messed that argument up anyway.
Then its image syntax of , that’s just plain nonsense.
I’d say that reStructuredText (from a little earlier) matched existing conventions slightly better than Markdown, in general. And it was much more sane, as a language, especially when you wanted to extend it.
> It’s meant to be human readable first and only incidentally convertible to actual markup.
If it were incidental, you wouldn’t use it—just go informal and write what suits you, it will be nicer.
No, the only reason for Markdown is so that you can write HTML with a nicer syntax. It’s not incidental at all. Sure, that syntax is intended for human-readability and -authoring, but it’s essential to Markdown that it is actual markup, poorly-defined though it be as a family of lightweight markup languages <https://en.wikipedia.org/wiki/Lightweight_markup_language>.
> Plain text is the presentation format
Do you know how many README.md files there are on GitHub that are essentially HTML soup? It’s a bit sad, really. It doesn’t invalidate the intention of what you’re saying, but I would say that Markdown is noticeably less about plain text being the presentation format than it was twenty years ago.