That's not how it works. W3C has generally been the codifer of standards with member agreement, W3C generally has not made a standard ahead of an implementation.
Granted, XHTML was probably an example, an exception that proves the rule.
No. The browser vendors left the W3C. When the final browser vendor (Microsoft) left and joined the other vendors in WHATWG, the vendors collectively sent a letter to W3C asking W3C to please stop copying and introducing errors into the standards WHATWG wrote. See e.g. https://www.zdnet.com/article/browser-vendors-win-war-with-w...
Browser vendor here: We're definitely still in the W3C and collaborating in a number of areas there. The WHATWG situation is complicated, but we have not "left the W3C".
Granted, XHTML was probably an example, an exception that proves the rule.