Browser support for URLs beginning with double slash

This behavior was part of RFC 1808 (Section 4) which is about 16 years old, so every major browser should (and does) support this.

Sadly, there's a bug with IE7 and -8 that will make them download the resources twice if a protocol-relative URL is used on a link or @import - which shouldn't be a big problem, but is ugly and should be kept in mind.

If you are developing on a local machine there's possibility it will fail with src="file://host.com/filename".

In this situation you need to specify scheme explicitly: http://host.com/filename or https://host.com/filename.