While messing around with some of the options over on the "Syntax for divs" issue in GitHub, I ran into some unexpected behavior with inserting horizontal rules immediately after text. All of the following create horizontal rules when preceded by a blank line:

___
***
---

They'll also work with spaces inserted between the characters. Of course if the three hyphens immediately follow text, they create a setext-style header. However the other two sequences shouldn't. Nor should any of the forms with inserted spaces. And both the other sequences and the three sequences with inserted spaces should also create a horizontal rule. Yet none of them do, unless they are preceded by a blank line. That is, none of the following generate horizontal rules under pandoc unless the "from" format is commonmark:

Hi
***

Hi
___

Hi
- - -

Hi
* * *

Hi
_ _ _

Isn't this standard markdown formatting?