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The term "globally unique identifier" is used in HTML and URL: https://html.spec.whatwg.org/multipage/browsers.html#origin https://url.spec.whatwg.org/#origin but is not defined. The submitted of a patch implementing this in Servo assumed it is a UUID, a randomly-generated string like "de305d54-75b4-431b-adb2-eb6b9e546014", but that seems wrong to me. HTML also defines: > Opaque identifiers > Internal values, with no serialisation, for which the only meaningful operation is testing for equality. Are globally unique identifiers the same as opaque identifiers? Either way, I’d like the spec to clarify this. Bug 26722 seems relevant, but I don’t understand its resolution. See also: https://github.com/whatwg/url/issues/21 https://github.com/servo/rust-url/pull/111
An opaque identifier is just some kind of identifier. It's not exposed to JavaScript, so what exactly it is doesn't really matter. Same for globally unique identifier. What matters is that it's an identifier that's globally unique, but since it's not exposed its format does not matter. It's only used for comparison purposes here and there. I can't really figure out a way how to explain that succinctly. Ideas?
I think the latest wording in https://github.com/whatwg/html/pull/925 makes this sufficiently clear. I'll update URL once that PR lands.