I've read some articles before that stated you can skip the scheme from your URLs and it'd be automatically determined from the page you're visiting. For example, if you're on https://test.com/ and you'd have an image like //google.com/logo.png, the image would be requested through a secure connection. Unfortunately, I can't find any reference right now (damn you, bookmarks!).
I tried it in Chrome and it worked. Tested on https://google.com:
(function (document) {
var img = document.createElement('img');
img.src = '//www.paypal.com/en_US/i/logo/paypal_logo.gif';
document.body.appendChild(img);
})(document);
Inspecting the elements, it brings in the logo from https://www.paypal.com.
I've skimmed the URL RFC but haven't found anything that clearly states this behavior. Does anyone know what the browser support for this feature is? I'm especially interested in mobile browsers.
