"Guidelines for the Use of XML within IETF Protocols" is currently in draft stage but it is still useful for XML application developers.

The guideline can be summarized as "keep it simple, stupid" or KISS. Avoid processing instructions, entity declarations and references; be careful with namespaces; beware of whitespaces; think twice before using attributes or even worse their default values, and so on.

When will W3C provide a guideline on what processing XML means?