URL to inherit protocol

I read somewhere that there's a trick you can use when linking images, that if you did ://d开发者_开发百科omain.com/images/s.gif it would inherit the protocol that's currently being used, whether it be https or http. This isn't working and I can't remember where I read it, I think css-tricks.com, am I doing something wrong?

Drop the :. The syntax for protocol-relative URLs is

//domain.com/images/s.gif