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What and Why
The Web is an ever evolving environment, and Web pages themselves are steadily
blooming from static displays of data to interactive applications. "Dynamic
HTML" is an umbrella term encompassing several ways in which Web developers
can breathe life into pages which have traditionally been still portraits of
information.
The basic notion behind Dynamic HTML is quite simple: allow any element of a page to be changeable at any time. Sounds like a dream, but as with any simple plan, "God is in the details," as they say. In the olden days, you could only change content on a page via CGI. This required a server to perform the changes to the page and re-serve the entire page, modifications and all, back to the client. While workable, this process was quite slow, as it placed a burden on both network traffic and server processing time. With long delays between a user's action and an on-screen response, building effective Web-based applications was quite constricting.
With DHTML, the magic occurs entirely on the client-side. This means that page modifications should appear immediately following a trigger, such as a user selection. And, remember, the DHTML dream is that you can modify any aspect of the currently loaded page -- text styles, swapped images, context-sensitive forms and tables, and even the on-screen data itself.
It's worth noting here, then, that "Dynamic HTML," isn't really about HTML, the markup language. By and large, DHTML describes the abstract concept of breaking up a page into manipulable elements, and exposing those elements to a scripting language which can perform the manipulations. The degree, or fineness, to which these elements are defined and actionable is a function of DHTML's maturity. Because we're only seeing the second generation of browsers supporting DHTML (MSIE 5 and the upcoming Netscape 5 based on Mozilla), DHTML is still an evolving and at times, inconsistent set of tools.