While keys cut across hierarchy
Keys let developers test relations between different parts of documents, and can be used to check things like "if X appears here, Y must appear over here." Keys are commonly used to ensure uniqueness within particular categories as well.
Keys use a simple subset of XPath to define the relations between the pieces.
Not particularly integrated
Keys and keyrefs aren't bonded tightly to the type structure, and rely on element and attribute names rather than their types.
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