So, here's the story from A to Z. So just sit back in your chair right there and let me tell you a story about... ahhhh forget it. I'm not that witty, but here's the deal.
Once upon a time w3schools was not a good source of information. Their site was riddled with bad information and poor programming practice, but their site was (and still is) consistently ranked very high in many search results.
Enter a group of developers who were concerned about all of the bad information on that site (and there was a lot of bad info on that site). These people created a site called w3fools. They dedicated themselves to pointing out all of the inaccuracies on w3schools. The word spread. Soon we all knew not to trust w3schools anymore.
Then something else happened. w3schools listened. They realized they were being perceived as a joke by serious developers and were being hurt by this reputation. They implemented a report error form on every page and started correcting any bad information they had. Since then, w3fools has claimed victory and completely changed their tone about w3schools. Unfortunately, the damage had been done. Developers are very leery of any information coming from w3schools and prefer the official documentation.
At this point in history it is probably safe to link to and learn from w3schools, but the official documentation should always be the canonical reference.