While I agree that the W3Fools list of mistakes on the W3Schools site is heavily diluted with nitpicks, I think W3Fools makes a valid point that W3Schools is in

While I agree that the W3Fools list of mistakes on the W3Schools site is heavily diluted with nitpicks, I think W3Fools makes a valid point that W3Schools is inaccurate and out-of-date to the point of being misleading, because in my personal experience there have been more than one instance in which case something I had "learned" on W3Schools before I knew better lead to mistakes I didn't realize until I started regularly consulting other sites instead. I also don't see what you mean that it's a "stupid site that exists sorely [sic] as a protest against the existence of another site". If a hypothetical site did exist that was as bad as W3Fools claims W3Schools is (setting aside that you don't seem to think W3Schools is that bad), don't you think it's nice, or at least not stupid, to have a site you can point people to that explains what's wrong with the hypothetical bad site and what alternatives there are? Validating your HTML is like linting your JS, I agree that it'd be dumb to do it for it's own sake, and in fact I know HTML and JS well enough and have enough quirky personal preferences that I don't do it, but I believe it is in fact good practice, especially for beginners, because invalid HTML, like JS that doesn't pass lint, can be a sign of bad practices or mistakes.