via a forward from Dan Brickley to the semantic-web mailing list we learn from Ted Guild:
From: Ted Guild
Date: Thu, 27 Apr 2006 10:58:26 -0400
Subject: change of W3Cs XSLT service
We are retiring our XT based service that has been in use since June
of 2000. The Saxon based XSLT 2.0 replacement  should be backwards
All transformation uris to the old service will be redirected to the
new one, there should be no perceived change. Many of the higher
volume transformations were tested.
– Ted Guild
While there couldn’t be enough good things stated about XT, the original XSLT processor developed by none other than James Clark, the same can be said about Dr. Michael Kay’s Saxon processing engine. Suffice it to say, without the first the others would be less likely to have followed.
A tremendous amount of gratitude to both James Clark and to Dr. Michael Kay for all that they have collectively done to bring about such a marvelous programming language, with full support via two tremendous processors in XT and Saxon respectively.
Also, to Bill Lindsey who has continued to maintain the XT processor, keeping things up-to-date with the latest offerings in regards to EXSLT, JAXP, and TrAX as well as a consistent and ongoing list of fixes and improvements along the way.