XML in a Nutshell, 3rd Edition

Standards and Extensions

The SAX and DOM specifications, along with the various core XML specifications, provide a foundation for XML processing. Implementations of these standards, especially implementations of the DOM, sometimes vary from the specification. Some extensions are formally specified—Scalable Vector Graphics (SVG), for instance, specifies extensions to the DOM that are specific to working with SVG. Others are just kind of tacked on, adding functionality that a programmer or vendor felt was important but wasn’t in the original specification. The multiple levels and modules of the DOM have also led to developers claiming support for the DOM but actually supporting particular subsets (or extensions) of the available specifications.

Porting standards also leads to variations. SAX was developed for Java, and the core SAX project only defines a Java API. The DOM uses Interface Definition Language (IDL) to define its API, but various implementations have interpreted the IDL slightly differently. SAX2 and the DOM are somewhat portable, but moving between environments may require some unlearning and relearning.

Some environments also offer libraries well outside the SAX and DOM interfaces. Perl and Python both offer libraries that combine event and tree processing—for instance, permitting applications to work on partial trees rather than SAX events or full DOM trees. These nonstandard approaches do not make moving between environments easy, but they can be very useful.