Summary

In a perfect world, there would be only a single schema language that is able to describe any XML document and simple to learn. Sadly, that schema definition language hasn't been written yet, so there are multiple schema definition languages. Each has benefits-and drawbacks-that relate to the choices made by the designers. RELAX NG leans more heavily towards simplicity, although it is a complete language. The syntax is highly approachable and understandable without requiring you to read the entire specification. Because the specification is newer than DTD and W3C XML Schema, there is not much tool support for RELAX NG yet. However, this should improve as more developers see the benefits of this flexible XML schema definition language.