I don't get this. He's produced something that you find "really nice", leveraging a resource and thanking said providers of the resource. And still they get dumped on... for what?

And with a page that appears filled with nitpicks and a level of pedantry that doesn't really matter to people who're just starting out with HTML/CSS/whatever else is being served up by W3Schools, or looking to learn something quickly. Basics which can then be used to develop deeper skills, as is wonderfully demonstrated here, and build expertise in a manner similar to how most people progress - iteratively, over time.