Adopting XML: Tomorrow's Web

Universality: or, all this is very nice but will it work?

A client reads a DTD to decide how to interpret elements in a document...
It reads a stylesheet to decide how to display those elements...
But how does it find functionality?
- JavaScript/ECMAScript?
  
  It's reasonable to expect XML supporting clients to support ECMAScript, so that it should be possible to pass scripts which add functionality within the document. Efficiency costs? Limits on what you can do?
- Java?
  
  It's reasonable to expect XML supporting clients to support Java, so it should be possible to pass Java components or applets to add functionality.
- Using the Document Object Model (DOM)?
  
  Either of the above will interact with the client through the DOM; in principle it should be possible to write DOM-aware components in any other language, provided you can be sure of finding an appropriate environment at the client end.
- XSL-T?
  
  This is really another way of applying one of the above: a transform of the document could create or request an appropriate software component which could then be run. XSL-T also supports software 'hooks', but in practice this again depends on the appropriate environment at the client end.
Gecko, IE5 cannot present SMIL...