Though relative url is not that popular, in my case there’s lots of posts that containing such url which need to be oneboxed.
It turns out wmd consider such url as a plain text and show it as
<p>URL</p> in preview, but onebox expect it as
<a href="URL"></a>. If we fix this behavior, will it affect other kind of preview? ‘//’ might be considered a code comment in a snippet mode.