Can a URL start with "//"?

Possible duplicate:
Is it valid to replace with // in a <script src = "...">?

We have a site at http [s]: //A.example.com/, which links to some resources (JS and CSS) at http [s]: //A.example.com/

This makes sense and avoids some browser warnings if the same scheme is used when linking to resources as the main site A is used. Site A uses PHP, so we can programmatically determine the current scheme (http or https) and insert it into the page header.

However, we found that using the // scheme instead of the scheme also works.

Thus, site A can link to "//B.example.com/theme/main.css" without any problems in all browsers we tried, regardless of whether we go to site A using http or https.

My question is: is this a valid URL? This seems to work, but I cannot find anyone else to do this, and I cannot find examples of how this is suggested or recommended.