I'm not a huge fan of technical books that treat the reader like an idiot. Books aimed at beginners are a good thing, but not if they talk down to the reader or explain the same thing dozens of times when twice would have been sufficient for even the most confused beginner. O'Reilly books tend not to be that way, which is why I almost always go with an O'Reilly manual over any other publisher.
I was skeptical when I opened this book and leafed through it. There are a lot of pictures and what appeared to be way too much whitespace for a technical manual. I thought that I was doomed to a condescending read. Thankfully, I was pleasantly surprised. The book was not only readable, it was a joy to read. I found myself picking it up at night rather than the novel I had picked up the day before. It is a great manual.
Note though, that Head First is not a reference manual. This is most definitely a tutorial. The book guides a beginner through the steps necessary to produce active content on a web page. Author Michael Morrison assumes that the reader has some knowledge of HTML and CSS, and I suspect that most readers would be totally lost without that prior knowledge. You don't need to be an expert, however. Some basic knowledge of HTML and CSS is enough.
Despite the fact that I was looking for more of a refresher course on material I already knew, I found myself reading this book to the end. At times it is funny, and it is always informative, and while it does repeat material several times, each repeat is presented in a different manner. It's obvious that Head First is aimed at different learning styles, and each time it repeats material, it is to present it to a different type of learner, and so it is presented differently. I really like the way Morrison moves through the material. There's a wonderful chapter on debugging (called "KIll Bugs Dead"), and a brief but informative introduction to AJAX.