Coralie Mercer at the World Wide Web Consortium blog, back in May 2019:
Today W3C and the WHATWG signed an agreement to collaborate on the development of a single version of the HTML and DOM specifications. The Memorandum of Understanding jointly published as the WHATWG/W3C Joint Working Mode gives the specifics of this collaboration.
That announcement is now reflected in this pull request at the WHATWG. The return to a single, shared specification for HTML and another for the DOM, is good news for developers, browser makers, and those of us who pitch in to make dev-friendly documentation over on MDN. It’s also important, I think, that the WHATWG “living standard” is what will live on, rather than the post-hoc versioned HTML 5.x specifications the W3C had been publishing.