The first version of DOM wasn’t an official specification, just the object model that Netscape Navigator 3 and Internet Explorer 3 implemented in their browsers. (Really, these were two different object models since they were only marginally compatible with each other.) This is sometimes called DOM Level 0.

DOM Level 0 only applied to HTML documents and only in the context of JavaScript. Nonetheless, both the usefulness of JavaScript and the growing incompatibility between the two browser object models made it obvious that something more standard was needed. Hence, the W3C launched the W3C DOM Activity and began working on DOM Level 1. DOM1 was an attempt to come out with a specification as quickly as possible that would codify existing practice while achieving some level of compatibility across browsers. Given the constraints the working group was laboring under, DOM1 is a surprisingly good spec. The naming conventions feel a little wrong to a Java programmer, but DOM1 does provides a solid core of functionality that covers maybe 75% of what programmers want to do when processing XML.

DOM Level 2 cleaned up the DOM Level 1 interfaces. The big change was namespace support in the Element and Attr interfaces. In addition, DOM2 added a number of supplementary interfaces for events, traversal, ranges, views, and style sheets. I’ll address these in upcoming chapters. In 2002, all significant XML parsers that support DOM, support DOM Level 2. There’s not a lot of reason to worry about the difference between DOM1 and DOM2. From this point forward, I’m just going to teach DOM2.

DOM Level 3 is visible not far up the road. Parts of it are just beginning to be supported by bleeding edge parsers, most especially Xerces 2. In the core, DOM3 just adds a few missing pieces needed to allow DOM to fully support all Infoset properties. This includes the original encoding and base URI of the document. However, DOM3 will also add some crucial functionality missing from DOM2. In particular, DOM2 doesn’t provide a parser-independent means to create a new Document object, either by parsing a file or by building one from scratch in memory. DOM3 will provide standard ways of doing both. DOM3 is also going to add a lot more support for DTDs and schemas. But despite all its new features and functionality, DOM3 will not replace DOM2. Everything that works today in DOM2 will continue to work the same way in DOM3. DOM3 extends the DOM into new territory, but it doesn’t change what has gone before.