Moderator Edit: The accuracy of w3schools.com has been challenged in another thread. In order to not pollute the original thread, the thread has been moved here.
Note to All: Please feel free to post about your experience in using w3schools.com, both good and bad.
Be a bit careful there though. That's just a commercial setup with a url which seems intended to give an impression of authority, or of connections with standards bodies, but actually has no status, and is often unreliable.
If you want something to rely on, the standards are at:
and Mozilla pages are also reliable: