https://html.spec.whatwg.org/multipage/ is the current HTML standard. It obsoletes all other previously-published HTML specifications.

As announced at https://www.w3.org/blog/2019/05/w3c-and-whatwg-to-work-together-to-advance-the-open-web-platform/, the W3C and the WHATWG signed an agreement to collaborate on the development of a single version of the HTML and DOM specifications:

• https://html.spec.whatwg.org/multipage/ is the single version of HTML being actively developed
• https://dom.spec.whatwg.org/ is the single version of the DOM specification being actively developed.

For further details about the W3C-WHATWG agreement, see the Memorandum of Understanding Between W3C and WHATWG.