[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.