1. A method of extending a definition of a first tag used in a first electronic document, wherein the first electronic document is encoded in a markup language, and the first electronic document is stored on a server in a computer network, the method comprising: defining the first tag in a first schema, wherein the definition of the first tag includes a plurality of elements from the markup language; defining a second tag in a second schema, wherein a definition of the second tag includes a reference to the first tag in the first schema, wherein the reference identifies the first tag as being extended; and an additional element from the markup language; and providing references for locating the first schema and second schema in a second electronic document, wherein the second tag is polymorphically used to encode text within the second electronic document; whereby the first and second schemas respectively allow use of the first or second tag and applications and document types that use the first or second tag can evolve separately; wherein the markup language is XML.