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Because XML lets you define your own markup languages, you can make full use of the extended hypertext features of XML (see the question on Links) to store or link to metadata in any format (eg using ISO 11179, as a Topic Maps Published Subject, with Dublin Core, Warwick Framework, or with Resource Description Framework (RDF), or even Platform for Internet Content Selection (PICS)).
There are no predefined elements in XML, because it is an architecture, not an application, so it is not part of XML's job to specify how or if authors should or should not implement metadata. You are therefore free to use any suitable method. Browser makers may also have their own architectural recommendations or methods to propose.
Id you are generating HTML5 from your XML, be aware that HTML5 does not use Dublin Core as HTML did (see http://www.w3.org/TR/html5/document-metadata.html) but it has a registration mechanism for extensions to the predefined set of metadata names.