As someone deep in the markup world, I'd say thank god XForms didn't make it, though I don't know Mozilla started WHATWG, let alone for promoting XForms of all things (Mozilla/Netscape once introduced JavaScript). WHATWG was Ian Hickson's (of Google) project, started out very successfully in adding capabilities to the web, but sadly turned into an instrument for browser monopoly.
As for validation, you can parse/validate all published versions of HTML, including W3C HTML 5, 5.1, and 5.2 using SGML and my SGML DTD grammar(s) [1] for HTML 5.x. SGML (ISO 8879) is the original markup meta-language on which both HTML and XML are based. XML was once started as a subset of SGML (dropping tag inference/omission, SGML-style empty elements, attribute shortforms, and other features requiring DTD declarations for the concrete markup language to parse).
The original W3C Workshop proposal to do Web Forms 2.0 was a Opera/Mozilla joint paper, and a few months later when the WHATWG started Apple had joined. I don't think Google's name was attached until Hixie moved to work there (admittedly not that much later).
