What's the difference between using "http://" and just "//" in src or href?

Question

I've seen some sites use //somedomain.com/images/img.jpg vs using http://somedomain.com/images/img.jpg which includes the http: as well.

Is there a difference between the two? Is the browser just correcting the missing http: and these people are being lazy? Im curious behind the reasoning.

Answer 1

If you are already on the site via http, it will assume you are talking about http and connect to the same server via the same protocol. Same with https. If you are on http and want to go to https, you would need to specify the protocol in the href.