And the fact that they are a good beginners resource doesn't make their own site's bad practices good practices. Even beginners know how to "view source".
The fellow who's so upset about their coding standards--I would suggest he visit their "Help us" page and the "Forum" page where he can contribute his expertise.
For a free, low-ad site, w3schools is remarkably useful. I hope this kind of article doesn't damage them too much. I'd be quite upset if they went away, or if they became more ad-heavy or subscriber-based to pay for all the fancy updates that he wishes they'd do.