Their stuff relating to JavaScript (the programming language) and the Document Object Model (the environment within which JavaScript is hosted in web browsers) is generally execrable.
Their reference materials (as opposed to tutorials) about those subjects are OK, but are just extracted from open source documentation of the subjects, so
http://developer.mozilla.org/ will give you a more recent version of the same material.
I haven't looked at their XML stuff. On the odd occasion that a Google search has led me to their XSLT reference, once again it's just been the XSLT docs chopped up into bite-size pieces, so a visit to
http://www.w3.org/TR/xslt and
http://www.w3.org/TR/xpath would have sorted me out anyway. Unfortunately Google still seems to have problems distinguishing between a site that recycles the standard that defines the technology, and one that gives you information about applying the technology effectively.
I vaguely remember finding something useful to do with PHP on there a while back, but the example code was pretty dreadful - full of security holes
Of course it's supposed to be an example of how to approach part of an overall solution, not something that provides a solution you can cut'n'paste. The word "school" in the URL indicates as much: I wouldn't expect to build a bridge on the basis of the examples of calculating beam bending under load that one might find being taught in a school.
Their JavaScript and DOM stuff is truly appalling though - well, all the bits of it I've ever seen are. The site is often cited as a primary reason why it's going to be so hard to educate people about how to do this stuff properly. I just went and had a look to see if it had improved recently, and the very first example they show uses the
document object's
write method. Wake up w3schools people: we stopped using that around the time we stopped supporting Netscape Navigator 3 and Internet Explorer 3
