Note that a url like //www.example.com will use the context's protocol, which means an img src="//www.example.com" will use http or https depending on which the page was loaded in. Very handy!
This is not Apache specific. If you take a look at the HTTP protocol, you will see why: Every request has the full path in it (e.g. GET /images/xyz.png HTTP/1.0) , but it is actually optional (in HTTP 1.0) to specify the domain name/host that you want to reach (most browsers send the Host header anyway, because serving multiple domains on one IP would be impossible otherwise).
