Some people traditionally not only underlined their headings, as in setext, but also overlined them. They never relied solely on the overline.
======== Heading ======== -------- Heading --------
Classic Markdown does not really support this, but for the second level, the preceding line of Hyphen-Minus characters
- is turned into a thematic break, i.e. a horizontal line,
<hr> in HTML. I believe parsers should have an option to suppress such breaks that directly precede a heading. This should probably not be part of the core specification, except that maybe lines of equal signs should also make a thematic break.
******** Heading ======== ________ Heading --------
PS: I have previously suggested that thematic breaks, including
= lines, could be reused for sectioning and pagination. I have also suggested a stricter generalization of markup characters that would also affect bullet lists.