Google XML Document Format Style Guide

This document provides a set of guidelines for general use when designing new XML document formats (and to some extent XML documents as well; see Section 11). Document formats usually include both formal parts (DTDs, schemas) and parts expressed in normative English prose.

This guide is meant for the design of XML that is to be generated and consumed by machines rather than human beings. Its rules are not applicable to formats such as XHTML (which should be formatted as much like HTML as possible) or ODF which are meant to express rich text. A document that includes embedded content in XHTML or some other rich-text format, but also contains purely machine-interpretable portions, SHOULD follow this style guide for the machine-interpretable portions. It also does not affect XML document formats that are created by translations from proto buffers or through some other type of format.
(Link: Google XML Document Format Style Guide)