In the DOM, documents have a logical structure which is very much like a tree; to be more precise, which
is like a "forest" or "grove", which can contain more than one tree. Each document contains zero or one
doctype nodes, one root element node, and zero or more comments or processing instructions; the root
element serves as the root of the element tree for the document. However, the DOM does not specify that
documents must be implemented as a tree or a grove, nor does it specify how the relationships among
objects be implemented. The DOM is a logical model that may be implemented in any convenient manner.
In this specification, we use the term structure model to describe the tree-like representation of a
document. We also use the term "tree" when referring to the arrangement of those information items
which can be reached by using "tree-walking" methods; (this does not include attributes). One important
property of DOM structure models is structural isomorphism: if any two Document Object Model
implementations are used to create a representation of the same document, they will create the same
structure model, in accordance with the XML Information Set [Infoset].