14. A browser comprising a computer program product tangibly stored in hardware, said computer program product being executable by computing equipment, said browser comprising: a markup rendering processor that is a runtime processor that renders or interprets markup documents; and a syntax processor that is a Document Type Definition (DTD) processor, wherein said browser is operable to: identify 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 browser; load the extender; and extend the markup language supported by the browser to include the new language tag, wherein the syntax processor uses the extender to dynamically produce an operational DTD based upon the linking instructions, wherein said operational DTD is used by the 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.
18. The browser of claim 14 , wherein said browser implements DOM event handling, and wherein said browser includes a step of adding an extension point defined by each of the plug-in packages as a Document Object Model (DOM) event.