It's interesting to see this IETF draft, inspired by the Atom category, taking the same route as the IPTC some time ago for the News Architecture and Newsml-G2.

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First Public Draft: Web Categories

Sam Johnston (ed), IETF Internet Draft

A version -00 I-D on "Web Categories" specifies the Category header-field for HyperText Transfer Protocol (HTTP), which enables the sending of taxonomy information in HTTP headers.

Summary: "A means of indicating categories for resources on the web has been defined by Atom (RFC 4287). This document defines a framework for exposing category information in the same format via HTTP headers. The 'atom:category' element conveys information about a category associated with an entry or feed. A given 'atom:feed' or 'atom:entry' element may have zero or more categories which must have a "term" attribute (a string that identifies the category to which the entry or feed belongs) and MAY also have a scheme attribute (an IRI that identifies a categorization scheme) and/or a label attribute (a human-readable label for display in end-user applications). Similarly a web resource may be associated with zero or more categories as indicated in the Category header-field(s). These categories may be divided into separate vocabularies or "schemes" and/or accompanied with human-friendly labels...

http://xml.coverpages.org/draft-johnston-http-category-header-00.txt