The document tree is the relational structure of an (x)html document. The tree structure of an (x)html document has many
aspects of a typical data tree structure, but also has some aspects which are unique to itself. In the structure, every element within
the document is contained within a parent element, except for the root element. For a standand xhtml document, the root element
is normally the
html element. This parent/child hierarchy of the xhtml tree is benificial to create and manipulate
the document tree structure, which Xport takes advantage of.