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

Question

I have the following element:

<script type="text/javascript" src="https://cdn.example.com/js_file.js"></script> 

In this case the site is HTTPS, but the site may also be just HTTP. (The JS file is on another domain.) I'm wondering if it's valid to do the following for convenience sake:

<script type="text/javascript" src="//cdn.example.com/js_file.js"></script> 

I'm wondering if it's valid to remove the http: or https:?

It seems to work everywhere I have tested, but are there any cases where it doesn't work?

Answer 1

A relative URL without a scheme (http: or https:) is valid, per RFC 3986: "Uniform Resource Identifier (URI): Generic Syntax", Section 4.2. If a client chokes on it, then it's the client's fault because they're not complying with the URI syntax specified in the RFC.

Your example is valid and should work. I've used that relative URL method myself on heavily trafficked sites and have had zero complaints. Also, we test our sites in Firefox, Safari, IE6, IE7 and Opera. These browsers all understand that URL format.