alt Hacker NewsMarkdown is for us humans. HTML is for machines.
Per keyboard stroke, you write much more content with MD than HTML.
Even without a specific browser/reader Markdown is relatively non-intrusive to read. You cant read HTML without an extra tool/effort to discard tags.
And you can parse MD with regular expressions, or can you? ;). (tip hat to infamous Stackoverflow discussion)
If HTML were for machines it would be a binary format. It's text so it's meant for humans to read and write, and humans did both for years without it being a problem.