Protocol agnostic/relative URLs are unneeded these days - let's just make them always HTTPS.
The guy who coined them, Paul Irish, says as much:
Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique is now an anti-pattern. If the asset you need is available on SSL, then always use the https:// asset.
Protocol agnostic/relative URLs are unneeded these days - let's just make them always HTTPS.
The guy who coined them, Paul Irish, says as much: