|Chapter 20 - Ten XSLT Processors Available Online|
|XSLT For Dummies|
|by Richard Wagner|
|Hungry Minds 2002|
msxsl does not provide support for extensions at this time. However, because Microsoft is a strong supporter of XML and XSLT technologies, I suspect you will see strong support for msxsl by Microsoft as well as see continued improvements in its capabilities in the future.
Available as a Windows executable.