Yes! The Caucho Resin server https://caucho.com/ https://caucho.com/ used / uses RelaxNG as its schema, and , amazingly flexibly, allowed for either attribute

Yes! The Caucho Resin server https://caucho.com/ https://caucho.com/ used / uses RelaxNG as its schema, and , amazingly flexibly, allowed for either attribute- or element-based configuration, the latter being much more pleasing to the eye in many cases. I don't think I've seen such flexibility anywhere else.