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Chapter 1. The State of the Art > Document Object Model

1.5. Document Object Model

When an HTML page loads into a scriptable browser, the browser creates a hidden, internal roadmap of all the elements it recognizes as scriptable objects. This roadmap is hierarchical in nature, with the most "global" object—the browser window or frame—containing a document, which, in turn, contains a form, which, in turn, contains form elements. For a script to communicate with one of these objects, it must know the path through the hierarchy to reach the object, so it can call one of its methods or set one of its property values. Document objects are the "things" that scripts work with.

Without question, the most hotly contested competition between Navigator and Internet Explorer has been how each browser builds its internal roadmap of objects. This roadmap is called a document object model (DOM). When one browser implements an object as scriptable but the other doesn't, it drives scripters and page authors to distraction. A lot of authors felt the sting of this problem when they implemented image-swapping mouse rollovers in Navigator 3. They soon discovered that images were not scriptable objects in Internet Explorer 3, so their IE 3 users were getting script errors when visiting the sites and moving their mice across the hot images.


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