I learned from w3schools in the early 2000s, and helped instruct my high school web design course with it.
I wish I hadn't. There were many innacuracies, outdated advice, and over simplifications.
I came up on a slew of weird and inconsistent sources, and that bit me when discussing things in college. I often felt over confident because of these type of sites.
W3schools (used to at least) gave a very shallow intro to programming languages (PHP, ASP, etc), and I remember a professor basically laughing at me once because of that misinformation.
So, maybe others had experiences like mine.
EDIT
They also had these weird "certifications" for technologies that were basically really dumbed down quizzes. They were made to make people feel special, I think. Dunno, I did them all, and I almost got laughed out of class.