Although I can't speak for the OP, and you may be asking for a different reason, but double slashes are an acceptable way to write fully qualified URLs while st

Although I can't speak for the OP, and you may be asking for a different reason, but double slashes are an acceptable way to write fully qualified URLs while still being protocol agnostic (that is to say, if you're on a page that was served via HTTPS, the double slash URL implies using the HTTPS protocol). It let's you have fully qualified URLs, without having to worry about the protocol and browser whining about mixed security.