But that was a correct emdash! Not quite the same. In fact "A markdown editor -- that allows you to edit live." would be correct, though with a slightly differ

But that was a correct emdash! Not quite the same. In fact "A markdown editor -- that allows you to edit live." would be correct, though with a slightly different meaning, sorta treating the latter clause as a surprising bonus, as though you would expect the opposite if you only heard the words "a markdown editor".