The :translate-alist for the md backend has no entry for fixed-width, so it defaults to the parent org-e-html-fixed-width which uses
(format "<pre class=\"example\">\n%s</pre>" ... So elements ,---- | : like | : this `---- get the html style markup. Some of the markdown processors I use don't like this. pandoc ignores the element entirely. The github blob generator copies it as verbatim. I have been using (fixed-width . org-md-verbatim) instead, but I think this is still not quite right. (IIUC, proper markdown uses four spaces at bol for such elements while org-md-verbatim uses surrounding backtics.) ? Chuck