1. A method for extending markup supported by a browser comprising: identifying a Web browser that presents information written in a markup language, wherein said Web browser comprises a markup rendering processor that is a runtime processor that renders or interprets markup documents, and wherein said Web browser comprises a syntax processor that is a Document Type Definition (DTD) processor; identifying an extender comprising an extension module and linking instructions, wherein the extension module defines a new language tag that was not previously part of a markup language supported by the Web browser; loading the extender; and extending the markup language supported by the Web browser to include the new language tag, wherein the extending step further comprises: the syntax processor using the extender to dynamically produce an operational DTD based upon the linking instructions, wherein said operational DTD is used by the Web browser in parsing a markup document that includes the new language tag; wherein the identified extender further comprises a plurality of attributes, wherein said Web browser further comprises an extension loader for loading the extender at runtime, wherein the attributes of the extender define times at which the extension loader is to load the extender, wherein specifiable times via the attributes comprise at least two of “when the Web browser loads,” “when a markup document is parsed,” when a markup document is rendered,” and when specified Document Object Model (DOM) events occur.