Wikipedia:PRURL

But this does:

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.

Example:

In citation templates, whether Citation Style 1 or Citation Style 2, protocol-relative URLs may be entered as |url=//www.example.com/.

Although past consensus discussions agreed to use PRURLs, no consensus was decided about changing them on a site-wide basis such as with a bot (see 17 January 2014 close box below). Implementation has been on an individual article and editor basis.

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.

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