What's the advantage of protocol-less urls over https by default?

Question

As the title states, why should I use // instead of https? I understand that on an https page you shouldn't load resources over http, but AFAIK there is no downside of loading your resources over https when the page itself is loaded over http. So what are the advantages of using // over https?

Answer 1

Use of https may be less efficient, since it involves encryption. A resource might reside on a web server that does not support https requests.