Some historical context: WHATWG[1], the Web Hypertext Application Technology Working Group, was originally a spin-off of the W3C[2], and was an organisation formed in 2004 by W3C members unhappy with decisions made in the W3C at that time. In 2019, a 'Memorandum of Understanding' was signed[3] agreeing to various principles for coordination between the two organisations.

The W3C is still very much active, and produces the vast majority of specifications that are implemented by 'web browsers' (in the general sense) like Chrome, Firefox and Safari, and they have their own standardisation process which is comparable in quality to organisations like ISO.

The area in which WHATWG are most active, though, is HTML, for which they produce what they call 'Living Standards' that have a different process from W3C's. CSS is done inside the W3C, and JavaScript is formalised by ECMA, an entirely different organisation still.

[1]: https://whatwg.org/

[2]: https://www.w3.org/

[3]: https://www.w3.org/2019/04/WHATWG-W3C-MOU.html

[4]: https://www.ecma-international.org