When you are considering how two things are alike, don't forget to also think about the ways in which they are not. Unlike with HTML, content producers who write Markdown, don't need their documents to be parsed correctly by many different engines. They only have to target the one they are using.
To be clear, "Standard Markdown" is only trying to address the ambiguity of the syntax. Sites are still going to implement Standard Markdown plus some ad hoc extensions, so it will still be the same practically in that regard.