NVDL can be very useful for everyone who works with comound documents. Those are XML documents which consist of multiple XML grammars, NVDL allows a straightforward schema-language neutral validation of such documents. In short you may specify which grammars (identified by their namespaces) may be combined in which contexts and which grammars you whish to validate agains a schema. First of all the NVDL engine checks if grammars occure in A correct contexts and after that it extracts various grammar fragments according to the NVDL schema and sends them to approprieate validators. I going to describe the whole mechanism and NVDL sematics in a greter detail in a separate document which will be published soon and the project page at SourceForge.
What is immportant, NVDL can be useful for many practical applications, Combining XML languages together bring many advatages. There are plenty of great presentation, semantic and descriptional languages and their combination can bring increadible possibilities. I good environment to demostrate this is the Web. Just imagine all those fancy Web designes achieved through bitmap fragments placed into renctabgles. It is much more effective (natural) to use vector graphics (SVG, SMIL) to achieve that. For a long time we all hear about the importance of semantic web. To achieve it's goals we need to add metadata into our pages, so again we need to use some kind of an embeded grammar. Another example is the great W3C XForms specification, which can significantly increase the rich-client user experience from using Web. Again XForms is a different grammar with a different namespace which needs to be embeded into XHTML or into a different container languages.
Combining different grammars can be quite error-prone. We need strict rules for doing so to ensure client application can handle all the different languages correctly. That's why validation of such documents is very important and it's sigificans will even rise in the future as more and more compound documents will be used in many different areas.