What is the effect of a double slash//Start URL in img?

I do not know if you have seen the following URL to the wording:

The URL in img starts with a double slash "//", which has a special purpose that determines whether the current page protocol is HTTP or HTTPS to determine the protocol that requests the URL. Prevent the "This Page Contains Both Secure and non-secure Items" error in IE.

But when you browse the local file, it resolves to the file://protocol. This feature can be used to load CDN:

Also applies to CSS:

. omgomg {Background:url (//lvtao.net/lvtao.gif);}

Note: If you use and @import to introduce CSS in IE7,IE8, you will download the CSS file two times.

Reference: http://www.paulirish.com/2010/the-protocol-relative-url/