Posted: July 19, 2004

Following several years of rapid development, during which time each new release of Internet Explorer (IE) brought with it a new API, the client side of Web application development has settled down, but has left in its wake a plethora of browser versions, APIs, and incompatibilities between versions of IE, as well as between IE and the standards set by various Internet standards bodies.

Since IE 3.0 in 1996, each version of Microsoft's Web browser has supported one or more Document Object Models (DOMs). A DOM is an API that allows client-side code, typically written in scripting languages such as VBScript or JScript, to read (and sometimes modify) the contents of a document through an object-oriented interface. A developer using an HTML DOM, for example, could access a table through a Table object with properties such as the number of rows and columns, rather than having to parse the text of the HTML page looking for <TR> and <TD> tags that define a table’s rows and columns. Each DOM defines, in effect, an API, and browsers often support multiple DOMs.

But with each new DOM, Web developers must weigh the benefits offered against the difficulty of writing code that works correctly across multiple browsers. As part of its efforts to standardize HTML, the World Wide Web Consortium (W3C) has categorized various DOMs according to functionality. (For a chart showing which DOMs are supported by IE, see "Web Client APIs".)

Level 0. The Level 0 DOM was first developed by Netscape and later adopted by Microsoft in IE 3.0. Although not formally standardized by the W3C, it has been used as the basis for later DOMs. The Level 0 DOM provides access to basic HTML features such as images and forms and provides enough functionality for simple "mouseover" scripts that change the contents of an image when the user hovers the mouse over it. In order to maintain backward compatibility, subsequent versions of IE have continued to support the basic DOM even as they have also included newer DOMs.

For Web developers seeking the greatest possible browser compatibility, the Level 0 DOM remains the only viable option, other than static HTML pages.

Intermediate DOMs. During the height of the "Browser Wars," Netscape and Microsoft each introduced browsers that supported interactive Web pages. Microsoft’s version was called Dynamic HTML (DHTML), and it provided user interface (UI) features previously only available from thick-client applications, and enabled new UI features such as drop-down menus. Outlook Web Access is an example of a fairly sophisticated Web application that uses DHTML to provide a UI experience almost as complete as that of a thick-client application. Although Microsoft's and Netscape’s implementations had many features in common, the two browsers were not compatible with each other, forcing Web developers to choose which browser to support, or to write code to detect the specific browser being used and respond accordingly.

Level 1. The W3C Level 1 DOM was originally developed for XML but was adapted for HTML. Although the Level 1 DOM became a W3C recommendation in Oct. 1998 (and was updated in Sept. 2000), it is not fully supported by versions of IE prior to 6.0. (IE 5.0 includes partial support of the Level 1 DOM.) Of the major alternate browsers, only Mozilla claims to support the Level 1 DOM; neither Opera nor Apple’s Safari browser claim compatibility. Once again, the difference in support for the DOM between versions of IE and between IE and competing browsers makes it all but impossible for Web developers to build sophisticated applications that work on all browsers. Some developers limit their applications by choosing a few selected browsers, while others bypass HTML and the DOM and opt instead for technologies like Macromedia’s Flash, which give them a known baseline on which to build their application.

Level 2 and beyond. Although Microsoft has been redirecting its development efforts away from the browser and back toward Windows and thick-client applications (see "No Stand-alone Updates on IE Roadmap"), the W3C has not been standing still. Two subsequent updates have been adopted as W3C Recommendations: Level 2 (adopted in Nov. 2000) extends the Level 1 DOM to provide additional support for Cascading Style Sheets (CSS), and Level 3 (adopted in Apr. 2004) contains further extensions for XML. Microsoft has not announced when or if it plans to adopt either of these extensions in IE. Mozilla claims to partially support the Level 2 DOM.