(Context: Using ChromeOS)

The biggest flaw I see in this (and in several alternatives suggested below) is that it doesn't support native browser spell check because it uses Ace (and in the case of alternatives, they do text rendering that isn't native and isn't detected as text). I use Markdown for taking outline based notes - and one of the biggest stopgaps is that I can spell check my work to make sure I'm not screwing up my own notes as I type during a lecture.

In that regard, the only thing I've found that meets this criteria isn't a Markdown editor, it's Writebox -- https://write-box.appspot.com/