Basically if you are a web developer then you learnt the basics from w3schools.com. Isn’t it? If you really followed w3schools.com then you should avoid w3schools.com right now. I also loved it but recently when I discovered w3fools.com (suggested by experienced web developer worldwide) from that time I just forget about w3schools.com. Really it may misleads you while you are learning web development basic tutorial from there. There are many many folks about what’s wrong in w3schools.com. To read those bullshit things you can go http://www.w3fools.com

The certificate given by w3schools is highly debatable. There is no value of this certificate in professional bodies.

w3schools has no support from governing standards bodies
Also w3schools has mentioned the following lines in their site’s footer.

We do not warrant the correctness of w3schools content. The risk from using it lies entirely with the user.
also w3schools.com teach users deprecated things like mysql_ function for php programming which is vulnerable to use. Also there are lots of misleading and wrong things on w3schools.com to see few of those wrong things please see W3fools. Also there is no contact between w3c and w3schools.

So I would highly recommend you to avoid w3schools.com from your learning source list. Happy learning!!