That's similar to other problems I have with sites recently: their super-special javascript on textareas screws up my own resizing of them, and even worse, ruins ctrl/cmd Z! I don't know how they do it, but screwing up Undo is pretty serious.
Somebody probably just copied a script from w3schools and smashed their face into it for a few hours until it did some approximation of what they might have maybe wanted it to do.
Unfortunately, the buckshot approach to software development leaves a lot to be desired. Especially when you start with such a horrible original product as w3schools.
Edit: I just found this excellent SO link on the topic: http://stackoverflow.com/questions/4662304/online-html-css-j...