The Next Stage in Composition -- McIlroy

I remain unconvinced.

However I think that the question of the importance of XSL is perhaps more so related to the question of the ultimate importance of XML.

The key value of XSL is that it's contained within the family of XML specifications, and adheres to the XML syntax. As such, it is potentially able to offer two advantages that were never available to SGML. The first is the innate ability to tie the appearance aspects of the publishing process with the workflow and commercial aspects of the processes, in a single data stream. Standards like JDF, AdML and NewsML arose during the XML era, not the SGML era, and promise enormous workflow and business benefits.