XML is powerful but not too complicated that one cannot learn it in a few days. In response to this situation, a group of developers have formed the SML-DEV mailing list to develop the Simple Markup Language. Both are useful to Java developers, although for different reasons. Indeed, even though XML is a standard, there are small differences in how vendors implement it. To avoid being burned by the differences, Common XML defines a subset that works reliably with every parser.
Publishers use XML to manage large Web sites or other documents.
Some constructs in XML exist solely for publishing applications. Mixed content is a prime example. However, mixed content is a distraction for data applications. Indeed, XML parsers pass indenting and spaces to applications that ignore them. Most applications that use Java and XML fall into the e-commerce or eAI range, so they will benefit from the tight focus on a practical subset.