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 obsolet

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 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 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” https://1.example/path/to/foo” and it uses this knowledge to rewrite URLs inside the page, https://1.example/path/to/bar https://1.example/path/to/bar into bar, https://1.example/path/from https://1.example/path/from into ../from, https://1.example/baz https://1.example/baz into /baz, https://2.example/ https://2.example/ into //2.example, and http://1.example/ http://1.example/ into http://1.example http://1.example.