Dave Ward says,
It’s not exactly light reading, but section 4.2 of RFC 3986 provides for fully qualified URLs that omit protocol (the HTTP or HTTPS) a
The question of whether one could change all their links to be protocol-relative may be moot, considering the question of whether one should do so. According to Paul Irish:
2014.12.17: 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.