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The HTML 2.0 specification, RFC1866, defines an SGML application and an Internet media type. The specification notes that extensions are planned, but only the text/html; level=2 internet media type and the "-//IETF//DTD HTML 2.0//EN" document type are defined. This document suggests the use of URIs as system identifiers for document type definitions, allowing decentralized evolution of the language. The use of marked sections as a transition technique and the continued use of the level mechanism for standardized points in the evolution path are discussed.