As mentioned earlier, I set out to define an alternative to the W3C's Document Object Model, inspired heavily by Java's XOM, but made for Common Lisp.
The result is STP, a data structure for XML that is full-featured and uses CLOS, but is more natural than DOM and gets namespaces right. Its implementation cxml-stp is available as an add-on library for Closure XML.
(For most purposes, it should be preferable to other alternatives, but DOM fans -- in case there are any -- can be assured that DOM support in cxml will not go away either.)
Read more about STP in the tutorial.