XML standardizes only a syntax, but if you constrain XML documents directly in terms of the sequences of characters that represent them, the syntactic noise is deafening. On the other hand, if you use an abstraction that incorporates concepts such as object orientation that have no basis in syntax, then you are coupling your XML processing components more tightly than necessary. What then is the right abstraction? The W3C XML Infoset Recommendation provides a menu of abstractions, but the items on the menu are of wildly differing importance.(via the quote of the day from Cafe con Leche)I would argue that the right abstraction is a very simple one. The abstraction is a labelled tree of elements. Each ment has an ordered list of children in which each child is a Unicode string or an element. An element is labelled with a two-part name consisting of a URI and local part. Each element also has an unordered collection of attributes in which each attribute has a two-part name, distinct from the names of the other attributes in the collection, and a value, which is a UNicode string. That is the complete abstraction. The core ideas of XML are this abstraction, the syntax of XML, and how the abstraction and syntax correspond. If you understand this, then you understand XML.
--James Clark
Read the rest in RELAX NG, O'Reilly & Associates, 2003, pp. ix-x
