In brief, it's the observation by Alan Flavell that new sites offering HTML tutorials fail miserably when submitted to validation or markup checkers. See http://ppewww.ph.gla.ac.uk/~flavell/www/ for the author's own description of the Law's genesis.
The question is, who cares? Some kid is proud enough to believe that his first attempts at HTML are a tutorial, and is innocent enough to announce this "tutorial" to the world at large. Kids are like that, you know. It doesn't mean we should judge him by our professional standards. Instead, we should judge the site as what it really is: A first attempt by a very enthusiastic kid. If anything, we should congratulate him on his efforts, discreetly point out a few of the more blatant errors, and gently nudge him in the right direction. Everything else is just sheer abuse of our superior knowledge. Matthias Gutfeldt
Unless the author is an adult and then you can point the finger and laugh because they should know better.