77Chapter 14 .Document Object Model Essentials only to HTML. The HTML portion inherits all the features of the Core DOM, while providing a measure of backward compatibility to object models already implemented in legacy browsers and providing a framework for new features. It is important for veteran scripters to recognize that the W3C DOM does not specify all features from existing browser object models. Many features of the Internet Explorer 4 (and later) object model are not part of the W3C DOM specification. This means that if you are comfortable in the IE environment and wish to shift your focus to writing for the W3C DOM spec, you have to change some practices as highlighted in this chapter. Navigator 4 page authors lose the tag (which is not part of HTML 4.0 and likely will never see the light of day in a standard) as well as the layer object. In many respects, especially with regard to Dynamic HTML applications, the W3C DOM is an entirely new DOM with new concepts that you must grasp before you can successfully script in the environment. By the same token, you should be aware that whereas NN6 goes to great lengths to implement all of DOM Level 1 and most of Level 2, Microsoft (for whatever reason) features only a partial implementation of the W3C DOM through IE5.5. This is true even though Microsoft participated in the W3C DOM working group and had more than ample time to put more of the W3C DOM into IE version 5.5. DOM levels Like most W3C specifications, one version is rarely enough. The job of the DOM working group was too large to be swallowed whole in one sitting. Therefore, the DOM is a continually evolving specification. The timeline of specification releases rarely coincides with browser releases. Therefore, it is very common for any given browser release to include only some of the most recent W3C version. The first formal specification, DOM Level 1, was released well after NN4 and IE4 shipped. The HTML portion of Level 1 includes DOM Level 0. This is essentially the object model as implemented in Navigator 3 (and for the most part in Internet Explorer 3 plug image objects). Perhaps the most significant omission from Level 1 is an event model (it ignores even the simple event model implemented in NN2 and IE3). DOM Level 2 builds on the work of Level 1. In addition to several enhancements of both the Core and HTML portions of Level 1, Level 2 adds significant new sections on the event model, ways of inspecting a document s hierarchy, XML names- paces, text ranges, style sheets, and style properties. What stays the same By adopting DOM Level 0 as the starting point of the HTML portion of the DOM, the W3C provided a way for a lot of existing script code to work even in a W3C DOM-compatible browser. Every object you see in the original object model starting with the documentobject (Figure 14-1) plus the image object are in DOM Level 0. Almost all of the same object properties and methods are also available. More importantly, when you consider the changes to referencing other elements in the W3C DOM (discussed in the next section), we re lucky that the old ways of referencing object such as forms, form elements, and images still work. Had the working group been planning from a clean slate, it is unlikely that the document object would have been given properties consisting of arrays of forms, links, and images.
You need excellent and relaible webhost company to host your web applications? Then pay a visit to Inexpensive Web Hosting services.
This entry was posted
on Thursday, September 27th, 2007 at 7:09 am and is filed under Tomcat.
You can follow any responses to this entry through the RSS 2.0 feed.
You can leave a response, or trackback from your own site.