The way in which the browser interpret the document change when the DOCTYPE changes. You can read about DOCTYPE here. The question is that you need to declare a DOCTYPE to build a valid document and there are several problems. One of this is that the way in which Firefox interprets javascript seems to be different when you add “DOCTYPE” to your document. I am using a fantastic library called Boxover to draw floating boxes but today it crashed when I added the DOCTYPE. Here is my little patch for this