How to pass the Protocall relative resource links test?
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.[1] 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.
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.