that's not bytesaving, that's called protocol relative URLs, meaning if the linking page is https, //foo.com will be loaded as https://foo.com https://foo.com.

that's not bytesaving, that's called protocol relative URLs, meaning if the linking page is https, //foo.com will be loaded as https://foo.com https://foo.com. most google properties have https modes, this is a way to reduce the complexity of the page and whatever generates the html code.