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http://html-differences.whatwg.org/#development-model [[ standard is under continual development where bugs are fixed and new features are introduced over time. Features can also be removed from the specification if they lack implementor interest or are not being used by Web developers, or for other reasons. ]] this is pretty much the case for 5.1 as well so suggest adding same or similar for it. difference being that 5.1 will eventually be called 5.2 etc
https://github.com/whatwg/html-differences/commit/775786cf9d05358a8b7c7e198157e4d93b6901ca