As far as I know, Earmark (or rather EarmarkParser) says that it supports Gruber Markdown though it tries its best to also support GFM, but there are edge cases where it does not and also it has its own set of extensions that are listed in its Readme.

So, as always with Markdown, ExDoc uses its own flavor that works differently than all other Markdown flavors If you want things to look nice on github and on hexpm, using as basic Markdown features as possible is the way to go in my oppinion.

Thanks. Digging a bit deeper, it seems that the Markdown is parsed correctly, but that ExDoc isn’t using it correctly. There are two issues. The first is that the character is quoted, so appears as ↩ There is an ExDoc issue for that. It was closed without a fix, but the person who raised it did produce a hack by overriding class="reversefootnote":

  <style>
    a.reversefootnote {
      display: inline-block;
      text-indent: -9999px;
      line-height: 0;
    }

    a.reversefootnote:after {
      content: ' ↩'; /* or any other text you want */
      text-indent: 0;
      display: block;
      line-height: initial;
    }

That leaves the problem of the link to the footnote, which Github renders as a superscripted character in brackets:

Although the <a> has class="footnote", that class doesn’t style the text to make it look like a footnote:

That can be fixed with another style:

    a.footnote {
      font-size: 0.7em;
      vertical-align: super;
    }

… which seems to work, though that three lines is a non-negligible fraction of all the CSS I’ve ever written.