Beyond the "descriptive vs. procedural" distinction

There has come to be a consensus that the “procedural vs. declarative” distinction is useful, if only as a rough guide, in the design of markup languages. To understand how and why this is the case, we need to ask questions that are usually left unasked when this principle is proposed, such as “is it the model (the schema) that we consider to be descriptive, or the tagged document?” or, more deeply, “why do we validate our markup