Browser support for URLs beginning with double slash

I've recently seen a few links used without a protocol. It didn't seem too difficult to understand - I think it's a great idea and pretty intuitive. For those of you unaware, using a URL like //example.com/script.js will point to either http://example.com/script.js or https://example.com/script.js depending on whether or not the URL originates from a http or https URL. Including http scripts or im