It's not exactly the case that it's the superset of all variants currently implemented. In fact that doesn't really make sense; two implementations can just hav

It's not exactly the case that it's the superset of all variants currently implemented. In fact that doesn't really make sense; two implementations can just have contradictory behaviour so there is no superset (well, unless you allowed things like picking one of the variants at random, I suppose ;) In fact the process is iterative. When you find that implementations do different things, you make an educated guess about what's most likely to be compatible with existing content. For example if all browsers allow an arbitrary number of slashes between the scheme and the host part of a URL, it seems very likely that some existing content relies on that, and very unlikely that you are going to get all the browsers to change their implementation, so you standardise that behaviour. On the other hand if you find that, say, one browser allows a semicolon rather than a colon after the scheme, but no other implementation does, it's very unlikely that is required for compatibility, so you typically don't allow colon-after-scheme in the spec, and speak to the existing implementation about fixing their bug. The hard case is where there isn't a clear consensus about the required behaviour, especially when implementations are flatly contradictory. In that case you speak to people and find out whether they have some evidence that their current behaviour has good effects, and whether they are willing to change. Armed with this, and your ability to reason about what should be more compatible or otherwise desirable, you pick one behaviour and hope that people converge on it. Often, in these cases, time will pass and it will be clear that you made the wrong choice; in that case you go back and edit the spec to reflect whatever reality turned out to be. So the process is iterative and not perfectly algorithmic; the person doing the work has to apply their judgement to make decisions where there isn't a clear path forward. But the goal, at least, is clear; it's to have a single specification that is in-line with what's actually implemented in the real world, and what's needed to consume real-world content. In this sense the URL spec is already a success; for example it was used as the basis for the Rust url library, which is believed to be compatible enough to consider using not just in Servo, but also in Firefox in place of the legacy C++ parser.