JavaScript: The Definitive Guide

E.4 Dynamic HTML

Perhaps the most exciting new features in Navigator 4.0 fall into the category of "dynamic html". These are HTML and JavaScript enhancements that allow Web pages to have much more dynamic behavior.

• Navigator 4.0 supports new <LAYER> and <ILAYER> tags that allow HTML text and objects to be positioned at absolute coordinates within a window, and to be stacked on top of each other (hence the name "Layer"). Layers function as sub-documents that can be independently positioned, stacked, and hidden. They support some dramatic new forms of JavaScript animation, as well as giving Web-page designers pixel-level control over the contents of a page. Each Document object has a layers[] array listing the Layer objects it contains, and each Layer object has a document property that refers to the HTML document it contains. Layers can be dynamically created with the Layer() constructor.

• Navigator 4.0 supports standard "Cascading Style Sheets" (CSS) and also supports a variant known as "JavaScript Style Sheets" (JSS). JavaScript style sheets provide essentially the same functionality as cascading style sheets do, but allow specification of document styles using JavaScript syntax, instead of the special-purpose CSS syntax. Where CSS have a purely descriptive syntax, JSS are described with a programming language and thus have additional run-time flexibility. JavaScript style sheets are an entirely new way in which JavaScript is used in HTML documents. Besides being used in event handlers and <SCRIPT> tags, JavaScript can now be used in <STYLE> tags as well.

• Navigator 4.0 and JavaScript 1.2 support a much more flexible event handling scheme. There is a new Event object that contains properties that describe the details of an event. Event handlers are now all passed an Event object when they are invoked. In addition, there is a much larger set of event handlers, and a well defined event-handling hierarchy. It is possible, for example, for individual Layers to respond to mouse events and individual keystrokes that occur over them.

• Miscellaneous related new features include new Window methods, a new Screen object, and new properties of the Navigator object. The window methods allow a program to resize and re-position windows, bring up a Print dialog, activate the Forward and Back browser buttons, and so on. Because of the power of these new methods, most of them are restricted to trusted scripts that have been digitally signed as described above. The Screen object provides information about the size and color depth of the screen on which Navigator is running. This allows JavaScript applications to customize themselves based on available screen real-estate, for example. The Navigator object has new platform and language properties that allow JavaScript programs to customize themselves based on the current platform and on the user's preferred language.