In June 2015, Wikipedia did a "hard enable" of HTTPS meaning the site is 100% HTTPS. Because protocol relative is relative to the site where the URL is linked from, all protocol relative URLs on Wikipedia will render as HTTPS.
This means protocol relative links should no longer be used on Wikipedia, and existing protocol relative links should be converted to either the HTTP or HTTPS scheme. Existing protocol relative links where the underlying is http will break. For example this doesn't work:
-  (//americanbilliardclub.com/about/history/)
But this does:
-  (http://americanbilliardclub.com/about/history/)
A protocol-relative URL (PRURL) is the method for linking to a website that offers both HTTP and HTTPS, while HTTPS links should be used for HTTPS-only websites and HTTP links should be used for sites that don't support HTTPS at all. For example, the Internet Archive supports both protocols and thus a PRURL could be used when linking to the site. A PRURL automatically uses either HTTP or HTTPS depending on the user's browser settings.
*[//archive.org Internet Archive]
Protocol-relative URLs only make sense in hyperlinks in webpages. Once printed on paper, they are no longer protocol-relative. In other words, when a person reads
//www.example.com/ from the paper and types it in a web browser, the browser has no preceding protocol to mimic. In Internet Explorer 11, Firefox and Google Chrome on Windows, the browser always assumes HTTP.
- Wikipedia:Village pump (policy)/Archive 111#As WP uses HTTPS, should (some) external links, too? (Original consensus discussion. 17 January 2014)
- Wikipedia:Village pump (technical)/Archive 123#How should external protocol-relative links be implemented? (Follow-up on how to implement. 23 January 2014)
- Wikipedia:Village pump (policy)/Archive 115#Protocol Relative URLs (Follow-up on Internet Explorer issues. 18 September 2014)
- Wikipedia:Village pump (proposals)/Archive 127#RfC: Should we convert existing Google and Internet Archive links to HTTPS? (HTTPS for archive.org and Google. 15 November 2015)
- Wikipedia:Village pump (technical)/Archive 155#Protocol relative URLs (June 2017)
- Wikipedia:Wikipedia Signpost/2011-07-25/Technology report#Protocol-relative URLs coming
- Wikipedia will switch to HTTPS by default
- Enabling PRLs on test wiki
- Protocol Relative URL quick overview