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It's a protocol relative link. So if the page is served over http, it'll be an http link, and if the page is served over https, it'll be an https link. See http://en.wikipedia.org/wiki/Protocol_relative_URL#Protocol-...

And it breaks unexpectedly (and sometimes hilariously) when people use it in html email, it turns into a file:// "link".

You can't have external css or js in an html email. [1] Are you talking about links? Or image tags?

[1] Well you can, but no one will fetch them.