The standard HTML/XHTML document model is static. Once displayed on the browser, a document does not change until the user initiates some activity, like selecting a hyperlink with the mouse. The Netscape developers found that limitation unacceptable and built in some special features to their browser that let you change HTML document content dynamically. In fact, they provide two different mechanisms for dynamic documents, which we describe in detail in this chapter. Internet Explorer supports some of these mechanisms, which we'll discuss as well.
 Of course, you could embed animated GIFs or applets that dynamically update the display, but the underlying HTML document itself doesn't change.
We should mention that many of the features of dynamic documents have been displaced by plug-in browser accessories and, in particular, applets. Nonetheless, Netscape and Internet Explorer continue to support dynamic documents, and we believe the technology has virtues you should be aware of, if not take advantage of, in your HTML documents. [Section 12.1]