Hmm I've been using w3schools for quite some time now but I ALWAYS double check it with a third source, often MDN. I really like their straight to the point approach and I think they've been fixing some of the odd stuff lately. I don't really think it's really that bad if you already have some previous experience, it can work OK as a quick reference.
all that link says is that w3c is "not reputable". no evidence. For learning the basics in webdev (coming from C/C++) I've never had a problem with w3c.
Even if they have fixed all these instances since then, that all those errors were ever produced is a bad sign, especially when there were better alternatives even then.