I'm 90% certain that the origin of WHATWG was in late 2003 or early 2004 when Dave Hyatt (Ex-Mozilla, then with Apple) Ian Hickson (ex-Opera, ex-Mozilla, then at Google) Start Parmenter (ex-Mozilla, then at OSAF or Oracle, I think,) Brendan Eich (still with Mozilla,) and a couple other folks were at Mozilla's brand new closet of an office, talking about XHTML's shortcomings, advanced we'd made with XUL, and how the web could best move forward given the XHTML failure and challenges from new platforms. I was a fly on the wall for that conversation and then a fly in IRC for more conversations, but it has been ages and my memory's not what it was then, (and I'm too lazy to dig through 30 years of archived emails to verify) so take it with a grain of salt.