Your comment is accurate but I think its the fact that GitHub styles it in that way. I do wish it was possible to use markdown-types of elements in this way so that it could fallback on non-GitHub rendered web pages, or still look good in a text editor vs. HTML tags all over the place.
Maybe something like
^ Header
^^^
Collapsed
^^^
Similar to code blocks with the ``` syntax. I dunno.
Maybe something like
^ Header
^^^
Collapsed
^^^
Similar to code blocks with the ``` syntax. I dunno.