Historically, at least, it had a lot of examples that technically did what they were supposed to do, but in ways that you really, really, really shouldn't be doing them.

I knew many developers that used w3schools that thought it was official source because w3c, I know it's not completely the sites fault but they named the domain. That's one reason I dislike, additional the quality of w3schools is compared with mdn for beginners that are not interested into the details.