[mananaysiempre]

Note that //foo/bar.txt is a valid address (a “network-path relative URI reference”[1]). Those are rarely seen but are used sometimes when the same resource or snippet needs to be usable with both HTTP and HTTPS. (I think I first learned about this form while reading a text on gradual migration to HTTPS circa 2014.)

My impression is this form might actually be the original way of expressing network paths, what with UNC in Windows (\\server\share\file etc.) and POSIX carving out an exception specifically for two slashes at the beginning of a pathname (that is, foo//bar is the same as foo/bar and ///foo/bar is the same as /foo/bar, but //foo/bar may or may not be the same as /foo/bar).

[1] https://datatracker.ietf.org/doc/html/rfc3986#section-4.2

[chrismorgan]

On the web, and most of the rest of the internet as well for most practical purposes, WHATWG’s URL Standard is now the normative reference for URLs, and obsoletes IETF’s RFC 3986 and 3987. (Source: https://url.spec.whatwg.org/#goals.) There, this concept is called a scheme-relative URL (a much more sensible name): https://url.spec.whatwg.org/#scheme-relative-url-string. They’re not the most common, but they’re not especially rare, either. HTML compressors especially will normally support emitting URLs in this form—you tell the compressor “this file you’re compressing will be served from the URL https://1.example/path/to/foo” and it uses this knowledge to rewrite URLs inside the page, https://1.example/path/to/bar into bar, https://1.example/path/from into ../from, https://1.example/baz into /baz, https://2.example/ into //2.example, and http://1.example/ into http://1.example.