1260 Part V . Putting JavaScript to Work (Free web hosting music)
1260 Part V . Putting JavaScript to Work terms of the support its browser has for a variety of technologies. My definition covers a broad range, because DHTML is not really any one thing. Instead it is an amalgam of several technologies, each of which has a standards effort in varying stages of readiness. The key technologies are as follows: Cascading Style Sheets; Document Object Model (DOM); and client-side scripting. To this list I also admit recent advances in Extensible Markup Language (XML), which opens the door to author-generated, page-specific HTML extensions that don t rely on standards bodies or browser support. It will help your authoring skills if you have a little historical perspective on how the Web arrived at DHTML. For many years, the HTML standard was intended for the rendering of static content not much more than an electronic version of a printed page. The most interactive part of a page was a form, which included buttons to click and text boxes to fill in. But for anything to change on the page, the content had to be served up again from the host computer. Client-side scripting, as first implemented through JavaScript in NN2, opened the way for HTML pages to not only contain some smarts, but also control individual pieces of content on the page without fetching a modified page from the server. At first, only form elements were scriptable. Soon thereafter, images could be swapped, although the rectangular space for the image was fixed when the page loaded. More dynamism accrued to pages in NN4 by way of the layer, which acted like a borderless, transparent or opaque window that could contain its own HTML document content and be positioned anywhere on the page, including overlapping content on the main page or other layers. A layer s entire content could be modified without touching the rest of the page or other layers. But the real breakthrough in dynamism came in IE4, whose rendering engine per mitted any element to be modified, inserted, or removed on the fly, while the rest of the page reflowed its content quickly and automatically in response to the change. At the same time, an accepted standard for style sheets (Cascading Style Sheets) opened the way for scripts to modify the look of content already on the page. Text could change colors when a cursor rolled atop it by either adjusting the style sheet property associated with the text or changing the style sheet rule that applies to the text. Development activity at both Netscape and Microsoft eventually led to a stan dard for the Document Object Model as a way for scripts to control HTML content directly. Unfortunately, the browser makers frequently implemented first, and then tried to establish their implementations as standards. Sometimes the implementa tions were not as complete as the standards became, leaving the browsers in states that only partially implement the standards, while paying homage to legacy imple mentations. Netscape used the occasion of developing an entirely new code base for what became NN6 to try to sever some ties with the past. In many respects that browser represents the state of the standard art as implemented so far. Newest ver sions of IE, on the other hand, must try to cater to both the legacy implementation and the standards, creating a massive DOM implementation with significant overlap in functionality with different syntaxes. Thus, the result of proprietary explorations and industry standards is a choice of modern browsers that permit a wide range of dynamic activity on content that reaches the browser. Browsers that had started life as sleepy renderers of a tiny HTML vocabulary have grown into powerful front ends for server applications, if not self-contained applications of a sort that execute entirely on the client computer.
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