I'd say w3schools is actually much better for beginners than MDN. If you learn HTML, CSS or JavaScript, MDN will often include too much unimportant information. (Similar to how Wikipedia math articles are often inscrutable for beginners because they are littered with lots of advanced low-importance information.) Additionally, w3schools has more easy "try it" examples which MDN consideres too trivial to bother with.

I agree though that MDN is much better than w3schools for non-beginners.