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

According to "Request for Comments: 3986" (https://www.ietf.org/rfc/rfc3986.txt) relative urls are valid:

4.2. Relative Reference

A relative reference takes advantage of the hierarchical syntax

(Section 1.2.3) to express a URI reference relative to the name space

of another hierarchical URI.

relative-ref = relative-part [ "?" query ] [ "#" fragment ]

relative-part = "//" authority path-abempty

/ path-absolute

/ path-noscheme

/ path-empty

The URI referred to by a relative reference, also known as the target

URI, is obtained by applying the reference resolution algorithm of

Section 5.

A relative reference that begins with two slash characters is termed

a network-path reference; such references are rarely used. A

relative reference that begins with a single slash character is

termed an absolute-path reference. A relative reference that does

not begin with a slash character is termed a relative-path reference.

A path segment that contains a colon character (e.g., "this:that")

cannot be used as the first segment of a relative-path reference, as

it would be mistaken for a scheme name. Such a segment must be

preceded by a dot-segment (e.g., "./this:that") to make a relative-

path reference.