As most people who've been around the block a couple of times know by now: w3schools is bad news. It presents itself as an organisation, affiliated with W3C. Not explicitly, of course, but the name would have you believe they are.
Their goal is to make money, by selling these bogus certificates to naive would-be webdevs, which is, from an evolutionary standpoint, not that bad, but that's a different matter.
The main issue, IMO, is that we know w3schools is bad news, by freely allowing amateurs/ beginners/ enthousiasts (call them what you will) to post links to it, aren't we providing w3schools with the credibility they crave while at the same time undermining SO's credibility?
I've seen questions to blacklist w3school all together, and I understand why the site isn't blacklisted, or at least: I understand the reasoning behind the site not being blacklisted.
That said, couldn't w3schools be excluded from comments, in the same way as whathaveyoutried is? possibly with a link to w3fools, or at least the relevant tag-wiki, which should contain links to propper knowledge repos.
Take this question, for example the OP posted a link to w3schools in a comment, and both Spudley and myself then commented, pointing out that w3schools isn't a good reference, and redirected the OP to MDN. So the comment system does work, great, but it would, IMHO be more efficient if the OP were prevented to post the intial comment in the first place, and got to see both a link to w3fools and/or the tag-wiki. The OP would've been aware of his using a bad source, and the w3schools wouldn't have caused your reading this rant, nor would I've found the need to post this question here, either :P
Ready for your down-votes in 3-2-1 ;P