This is closely related to design goal #4 above; a data format is programmer-friendly if programmers can read and use the defining documents; otherwise not. Too many other standards and specifications have relied too heavily on prose and not enough on formalisms.
This is one area where some, in particular Dan Connolly, have argued that the actual XML spec falls short of the goal; that a version could have been created which was substantially more formal and concise than the current document.
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