Browser support for URLs beginning with double slash

For some reason, I keep forgetting if double slash URLs are supported in older browsers or not.  Maybe if I post the answer from this StackOverflow question here, I will remember it myself next time:

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.

So, that might be a problem for the CSS, but the JavaScript and images should work just fine.