I tried to leave some details here on the situation downthread: https://news.ycombinator.com/item?id=12849375

As for actual technical differences, the vast majority of them are bugs introduced during the copying process, or just places where they haven't caught up to our bug fixes/new features/removals of unimplemented features. (Bug fixes especially roll in quite frequently; see https://github.com/whatwg/html/commits/master .)

There are some differences around conformance requirements, as some of the W3C HTML editors see their fork as a venue for writing a nice official-looking documenton how they think certain HTML elements should be semantically interpreted and used. The big ones I remember are around figcaption, main, hgroup, and others. My favorite is when they made it a high priority to change one of the examples not to use the Flying Spaghetti Monster since it might be controversial. This does not affect implementations of browsers though, just what conformance checkers (validators) like http://checker.html5.org/ tell you about whether your document is conformant.

You can get a full comparison via the third-party tool https://diffofhtmls.herokuapp.com/.

Basically W3C tries to define versioned HTML specifications, but they basically follow what browsers actually implement and are always out of date. WHATWG maintain a "living standard" and is really what browser vendors actually work with because it doesn't attempt the slow and futile task of defining a 'stable' versioned spec.

Something like that anyway.

Further down is a comment by LukeShu that explains the difference a bit:

https://news.ycombinator.com/item?id=12849105

You already got many relevant answers but just in case you need it again, https://www.google.com/ncr redirects to the international version.