XML Spy - the first true Integrated Development Environment for XML

XML Spy is the first true Integrated Development Environment for the eXtensible Markup Language that includes all major aspects of XML in one powerful and easy-to-use product:

Their words not mine but looks impressive nevertheless. Interestingly, the XML Spy website is created from XML files:

All the pages you see here are created from XML files conforming to our own DTD, which gives us intelligent editing capabilities and validation while creating the site content. These XML files are then transformed into XHTML by XML Spy using the XSL Transformation batch processing command in the project view.

Shame it's Windows-only.

Perhaps someone can explain to me how in the XML SPY - features - lite: Premium/Lite Comparison a comparison is being made when:

This page details all the differences between the two versions of XML Spy. All the features listed on this page are only available in the Premium version of our product.