From Wikizine #130:
=Did you know …=… what protocol relative URLs are?
Normal URLs look like: http://test.wikipedia.org/wiki/Main_Page or https://test.wikipedia.org/wiki/Main_Page.
Both of these URLs define the protocol that will be used. Protocol relative URLs look like this: //test.wikipedia.org/wiki/MainPage. Dropping the protocol from the URL allows the browser to assign the current protocol to the URL. So, if you are visiting the site in HTTPS mode, links will point to HTTPS, and if you are visiting the site in HTTP mode, links will point to HTTP.
:* http://blog.wikimedia.org/2011/07/19/protocol-relative-urls-enabled-on-test-wikipedia-org – full post about this by Ryan Lane :* http://blog.wikimedia.org/2011/09/27/protocol-relative-urls-enabled-on-all-wikimedia-foundation-wikis/ – protocol relative URLs are now live