Network-Path Reference URI / Scheme relative URLs

Scheme relative URLs (network-path references) are something that I've just found out about - where you don't specify the scheme of a URL and it picks it up from the current context.

For example: <img src="//domain.com/img.png" /> will resolve to https://domain.com/img.png if the current scheme is HTTPS or http://domain.com/img.png if it is not.

This seems like a very easy way to resolve those pesky problems of calling an external script or image on an SSL page without bringing up the dreaded error that some content on a page is not secure.

The benefit seems obvious, but what I don't seem to be able to find is a huge amount of information on this and was wondering if anyone had any experience or references about scheme relative URLs (good or bad)?

Whilst I'm trying to discover if there are any browsers that this causes issues with (I've been successful with IE6-8, Chrome and Firefox), I'm also interested to find out if anyone has any experience using this in different languages. For example, would it work if you were to issue a Response.Redirect with a scheme relative URL in ASP?

//domain.com/img.png is a perfectly valid URI syntax as per RFC 3986: Section 4.2.

It is relative to the current scheme, and therefore as you mentioned, it can be very useful when switching between http and https, because you won't need to explicitly specify the scheme.

All modern browsers will understand that format, including IE 6.

Further reading on Stack Overflow:

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

• Using // in a <script>'s source

If you want to support IE 7 and 8, you might want to consider that scheme relative URLs for stylesheets will cause them to be downloaded twice:

> [...] if you try this in Internet Explorer 7 and 8 you’ll see that stylesheets specified with a protocol relative URL are downloaded twice. > > Developers should avoid using protocol relative URLs for stylesheets if they want their pages to be as fast as possible in Internet Explorer 7 & 8.

Source: http://www.stevesouders.com/blog/2010/02/10/5a-missing-schema-double-download/